Quinones having high capacity retention for use as electrolytes in aqueous redox flow batteries

ABSTRACT

We disclose quinone compounds and related species (Formula I) that possess significant advantages when used as a redox active material in a battery, e.g., a redox flow battery. In particular, the compounds provide redox flow batteries (RFBs) with extremely high capacity retention. For example, RFBs of the invention can be cycled for 500 times with negligible loss of capacity, and such batteries could be employed for years of service. Thus, the invention provides a high efficiency, long cycle life redox flow battery with reasonable power cost, low energy cost, and all the energy scaling advantages of a flow battery.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under DE-AC05-76RL01830,DE-AR0000348, and DE-AR0000767 awarded by the U.S. Department of Energyand under 1509041 awarded by the National Science Foundation. Thegovernment has certain rights to the invention.

TECHNICAL FIELD

This invention relates generally to energy storage. More specifically,the invention relates to a new class of compounds and their operation ina redox flow battery. The compounds are composed only of earth-abundantelements, having high electrochemical stability and high watersolubility.

BACKGROUND OF THE INVENTION

Redox flow batteries (RFBs) represent a class of energy storage devicesthat are especially suited for large-scale stationary deployment. Thecost of RFB electrolytes, which are the charge-storing materials,usually constitutes a large proportion of the cost of a complete RFBsystem. For other system components, large cost savings are realized byutilizing water as the solvent for the electrolytes and carbon for theelectrodes. Long term stability of the electrolytes is also important tothe commercial success of the battery.

Accordingly, there is a need for new redox active species having longterm stability for use in RFBs.

SUMMARY OF THE INVENTION

The invention features redox flow batteries and compounds useful thereinas negolytes or posolytes. The batteries and compounds are advantageousin terms of being useable in water solutions at neutral pH and haveextremely high capacity retention. Flow batteries based on thesematerials can store large amounts of energy. Because of thenon-hazardous nature of these compounds, this method of energy storageis safe for use in the large-scale electrical grid or for smaller-scaleuse in buildings. Flow batteries have scaling advantages over solidelectrode batteries for large scale energy storage. Batteries based onquinones can have high current density, high efficiency, and longlifetime in a flow battery. High current density drives downpower-related costs. The other advantages this particular technology hasover other flow batteries include inexpensive chemicals, energy storagein the form of safer liquids, an inexpensive separator, little or noprecious metals usage in the electrodes, and other components made ofplastic or inexpensive metals with coatings proven to afford corrosionprotection.

In one aspect, the invention features, a redox flow battery including afirst aqueous electrolyte comprising a first redox active material; anda second aqueous electrolyte comprising a second redox active materialthat is a compound of formula I:

or an ion, salt, or hydroquinone thereof,wherein each of R₁-R₈ is independently H; halo; optionally substitutedC₁₋₆ alkyl; optionally substituted C₃₋₁₀ carbocyclyl; optionallysubstituted C₁₋₉ heterocyclyl having one to four heteroatomsindependently selected from O, N, and S; optionally substituted C₆₋₂₀aryl; optionally substituted C₁₋₉ heteroaryl having one to fourheteroatoms independently selected from O, N, and S; oxo; —NO₂; —OR_(a);—N(R_(a))₂; —C(═O)R_(a); —C(═O)OR_(a); —S(═O)₂R_(a); —S(═O)₂OR_(a);—OS(═O)₂OR_(a); —P(═O)R_(a2); —P(═O)(OR_(a))₂; and —OP(═O)(OR_(a))₂;—X₁-L₁-C(O)O—Y₁; —X₂-L₂-C(O)O—Y₂; —X₃-L₃-P(═O)(OY₃)₂; or—X₄-L₄-P(═O)(OY₄)₂; wherein each R_(a) is independently H; optionallysubstituted C₁₋₆ alkyl; optionally substituted C₃₋₁₀ carbocyclyl;optionally substituted C₁₋₉ heterocyclyl having one to four heteroatomsindependently selected from O, N, and S; optionally substituted C₆₋₂₀aryl; or optionally substituted C₁₋₉ heteroaryl having one to fourheteroatoms independently selected from O, N, and S;X₁, X₂, X₃, and X₄ are independently O, S, or CH₂; L₁, L₂, L₃, and L₄are independently C₁-C₆ alkylene; and Y₁, Y₂, Y₃, and Y₄ areindependently H or optionally substituted C₁-C₆ alkyl, provided that oneand only one of R₁-R₈ is —X₁-L₁-C(O)O—Y₁ or —X₃-L₃-P(═O)(OY₃)₂ and oneand only one of R₁-R₈ is —X₂-L₂-C(O)O—Y₂ or —X₄-L₄-P(═O)(OY₄)₂.

In certain embodiments, X₁ and X₂ are O. In other embodiments, each ofR₁-R₈ is independently H; halo; optionally substituted C₁₋₆ alkyl;—X₁-L₁-C(O)O—Y₁; or —X₂-L₂-C(O)O—Y₂. In further embodiments, R₂ is—X₁-L₁-C(O)O—Y₁, and R₆ is —X₂-L₂-C(O)O—Y₂. In yet other embodiments, Y₁and Y₂ are H.

In certain embodiments, X₃ and X₄ are O. In other embodiments, each ofR₁-R₈ is independently H; halo; optionally substituted C₁₋₆ alkyl;—X₃-L₃-P(═O)(OY₃)₂; or —X₄-L₄-P(═O)(OY₄)₂. In further embodiments, R₂ is—X₃-L₃-P(═O)(OY₃)₂, and R₆—X₄-L₄-P(═O)(OY₄)₂. In yet other embodiments,Y₃ and Y₄ are H.

Exemplary quinones are

or an ion, salt, or hydroquinone thereof; or

or an ion, salt, or hydroquinone thereof.

For example, the quinone is

or an ion, salt, or hydroquinone thereof, or the quinone is

or an ion, salt, or hydroquinone thereof, or the quinone is

or an ion, salt, or hydroquinone thereof.

In certain embodiments, the quinone is

or an ion, salt, or hydroquinone thereof.

In further embodiments, the second redox active material is thehydroquinone of formula I, which is oxidized to the correspondingquinone during discharge. The pH of the second aqueous electrolyte is,for example, ≥7. In particular embodiments, the pH is from 8-13. Thefirst redox active material may include bromine, chlorine, iodine,oxygen, vanadium, chromium, cobalt, iron, aluminum, manganese, cobalt,nickel, copper, or lead.

In a related aspect, the invention features a compound of formula I:

or an ion, salt, or hydroquinone thereof,wherein each of R₁-R₈ is independently H; halo; optionally substitutedC₁₋₆ alkyl; optionally substituted C₃₋₁₀ carbocyclyl; optionallysubstituted C₁₋₉ heterocyclyl having one to four heteroatomsindependently selected from O, N, and S; optionally substituted C₆₋₂₀aryl; optionally substituted C₁₋₉ heteroaryl having one to fourheteroatoms independently selected from O, N, and S; oxo; —NO₂; —OR_(a);—N(R_(a))₂; —C(═O)R_(a); —C(═O)OR_(a); —S(═O)₂R_(a); —S(═O)₂OR_(a);—OS(═O)₂OR_(a); —P(═O)R_(a2); —P(═O)(OR_(a))₂; and —OP(═O)(OR_(a))₂;—X₁-L₁-C(O)O—Y₁; —X₂-L₂-C(O)O—Y₂; —X₃-L₃-P(═O)(OY₃)₂; or—X₄-L₄-P(═O)(OY₄)₂; wherein each R_(a) is independently H; optionallysubstituted C₁₋₆ alkyl; optionally substituted C₃₋₁₀ carbocyclyl;optionally substituted C₁₋₉ heterocyclyl having one to four heteroatomsindependently selected from O, N, and S; optionally substituted C₆₋₂₀aryl; or optionally substituted C₁₋₉ heteroaryl having one to fourheteroatoms independently selected from O, N, and S;X₁, X₂, X₃, and X₄ are independently O, S, or CH₂; L₁, L₂, L₃, and L₄are independently C₁-C₆ alkylene; and Y₁, Y₂, Y₃, and Y₄ areindependently H or optionally substituted C₁-C₆ alkyl, provided that oneand only one of R₁-R₈ is —X₁-L₁-C(O)O—Y₁ or —X₃-L₃-P(═O)(OY₃)₂ and oneand only one of R₁-R₈ is —X₂-L₂-C(O)O—Y₂ or —X₄-L₄-P(═O)(OY₄)₂.

In certain embodiments, when R₂-R₄ and R₆-R₈ are H, R₁ and R₅ are notboth —SCH₂C(O)OH, —OCH₂C(O)OH, or —OCH₂C(O)OCH₂CH₃; when R₄ and R₈ are Hand R₂, R₃, R₆, and R₇ are —O(CH₂)₃(CH₃), R₁ and R₅ are not both—O(CH₂)₃C(O)OH; when R₄ and R₈ are H and R₂, R₃, R₆, and R₇ are—O(CH₂)₄(CH₃), R₁ and R₅ are not both —O(CH₂)₄C(O)OH; when R₁ and R₈ areOH and R₃-R₆ are H, R₂ and R₇ are not both —(CH₂)₃C(O)OH or—(CH₂)₃C(O)OCH₃; when R₁, R₄, R₅, and R₈ are NH₂ and R₃ and R₆ are H, R₂and R₇ are not both —O(CH₂)₃C(O)O(CH₂)₄CH₃; when R₁, R₃-R₅, R₇, and R₈are H, R₂ and R₆ are not both —OCH(CH₃)C(O)OCH₃, —OCH(CH₃)C(O)OH, or—OCH₂C(O)OCH₃; when R₁, R₃, R₅, and R₇ are H, and R₄ and R₈ are—N(R_(a))₂, R₂ and R₆ are not both —S(CH₂)_(n)C(O)OCH₂CH₃, wherein n is1 to 5; when R₁ and R₄ are OCH₃, and R₅-R₈ are H, R₂ and R₃ are not both—CH₂CH₂C(O)OH, —CH₂CH₂C(O)OCH₃, or —CH₂CH(C(O)CH₃)C(O)OCH₂CH₃; when R₁and R₄ are OH, and R₅-R₈ are H, R₂ and R₃ are not both —CH₂CH₂C(O)OCH₃;when R₁ and R₄-R₈ are H, R₂ and R₃ are not both—CH₂C(NHC(O)CH₃)(C(O)OCH₂CH₃)₂ or —CH₂CH(NHC(O)CH₃)C(O)OCH₂CH₃; whenR₂-R₇ are H, R₁ and R₈ are not both —OCH₂C(O)OCH₂CH₃ or —SCH₂CH₂C(O)OH;when R₂ and R₄-R₇ are H and R₃ is —CH₃, R₁ and R₈ are not both—OCH₂C(O)OCH₂CH₃; when R₂, R₃, R₆, and R₇ are H and R₄ and R₅ are—N(R_(a))₂, R₁ and R₈ are not both —OCH₂C(O)OCH₂CH₃ or —OCH₂C(O)OH; whenR₂-R₄, R₇, and R₈ are H and R₅ is NH₂, R₁ and R₆ are not both—SCH₂C(O)OH; when R₂-R₆ are H and R₈ is NH₂, R₁ and R₇ are not both—SCH₂C(O)OH; when R₃-R₈ are H, R₁ and R₂ are not both —OC(O)CH₂CH₂C(O)OCH₂CH₃; when R₁, R₃-R₅, R₇, and R₈ are H, R₂ and R₆ are not both—OC(O)CH₂CH₂C(O)O CH₂CH₃; when R₂, R₃, and R₅-R₈ are H, R₁ and R₄ arenot both —OC(O)CH₂CH₂C(O)OCH₂CH₃ or —SCH₂C(O)OH; when R₂-R₇ are H, R₁and R₈ are not both —OC(O)CH₂CH₂C(O)OCH₂CH₃; when R₂, R₃, R₆, and R₇ areH and R₄ and R₅ are —(CH₂)₅CH₃, R₁ and R₈ are not both —(CH₂)₇C(O)OH;when R₂, R₃, R₆, and R₇ are H and R₄ and R₈ are —(CH₂)₅CH₃, R₁ and R₅are not both —(CH₂)₇C(O)OH; and/or when R₃ and R₆ are H and R₁, R₄, R₅,and R₈ are —NH₂, R₂ and R7₅ are not both —O(CH₂)₃C(O)OCH₃.

In certain embodiments, X₁ and X₂ are O. In other embodiments, each ofR₁-R₈ is independently H; halo; optionally substituted C₁₋₆ alkyl;—X₁-L₁-C(O)O—Y₁; or —X₂-L₂-C(O)O—Y₂. In further embodiments, R₂ is—X₁-L₁-C(O)O—Y₁, and R₆ is —X₂-L₂-C(O)O—Y₂. In yet other embodiments, Y₁and Y₂ are H.

In certain embodiments, X₃ and X₄ are O. In other embodiments, each ofR₁-R₈ is independently H; halo; optionally substituted C₁₋₆ alkyl;—X₃-L₃-P(═O)(OY₃)₂; or —X₄-L₄-P(═O)(OY₄)₂. In further embodiments, R₂ is—X₃-L₃-P(═O)(OY₃)₂, and R₆ is —X₄-L₄-P(═O)(OY₄)₂. In yet otherembodiments, Y₃ and Y₄ are H.

Exemplary compounds are

or an ion, salt, or hydroquinone thereof, or

or an ion, salt, or hydroquinone thereof.

For example, the compound is

or an ion, salt, or hydroquinone thereof, or the compound is

or an ion, salt, or hydroquinone thereof, or the compound is

or an ion, salt, or hydroquinone thereof.

In certain embodiments, the compound is

or an ion, salt, or hydroquinone thereof.

By “alkyl” is meant straight chain or branched saturated groups from 1to 6 carbons. Alkyl groups are exemplified by methyl, ethyl, n- andiso-propyl, n-, sec-, iso- and tert-butyl, neopentyl, and the like, andmay be optionally substituted with one or more, substituents.

By “aryl” is meant an aromatic cyclic group in which the ring atoms areall carbon. Exemplary aryl groups include phenyl, naphthyl, andanthracenyl. Aryl groups may be optionally substituted with one or moresubstituents.

By “carbocyclyl” is meant a non-aromatic cyclic group in which the ringatoms are all carbon. Exemplary carbocyclyl groups include cyclopropyl,cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl.Carbocyclyl groups may be optionally substituted with one or moresubstituents.

By “halo” is meant, fluoro, chloro, bromo, or iodo.

By “heteroaryl” is meant an aromatic cyclic group in which the ringatoms include at least one carbon and at least one O, N, or S atom,provided that at least three ring atoms are present. Exemplaryheteroaryl groups include oxazolyl, isoxazolyl, tetrazolyl, pyridyl,thienyl, furyl, pyrrolyl, imidazolyl, pyrimidinyl, thiazolyl, indolyl,quinolinyl, isoquinolinyl, benzofuryl, benzothienyl, pyrazolyl,pyrazinyl, pyridazinyl, isothiazolyl, benzimidazolyl, benzothiazolyl,benzoxazolyl, oxadiazolyl, thiadiazolyl, and triazolyl. Heteroarylgroups may be optionally substituted with one or more substituents.

By “heterocyclyl” is meant a non-aromatic cyclic group in which the ringatoms include at least one carbon and at least one O, N, or S atom,provided that at least three ring atoms are present. Exemplaryheterocyclyl groups include epoxide, thiiranyl, aziridinyl, azetidinyl,thietanyl, dioxetanyl, morpholinyl, thiomorpholinyl, piperazinyl,piperidinyl, pyrrolidinyl, tetrahydropyranyl, tetrahydrofuranyl,dihydrofuranyl, tetrahydrothienyl, dihydrothienyl, dihydroindolyl,tetrahydroquinolyl, tetrahydroisoquinolyl, pyranyl, pyrazolinyl,pyrazolidinyl, dihydropyranyl, tetrahydroquinolyl, imidazolinyl,imidazolidinyl, pyrrolinyl, oxazolidinyl, isoxazolidinyl, thiazolidinyl,isothiazolidinyl, dithiazolyl, and 1,3-dioxanyl. Heterocyclyl groups maybe optionally substituted with one or more substituents.

By a “nitrogen protecting group” is meant those groups intended toprotect an amino group against undesirable reactions during syntheticprocedures. Commonly used nitrogen protecting groups are disclosed inGreene, “Protective Groups in Organic Synthesis,” 3^(rd) Edition (JohnWiley & Sons, New York, 1999), which is incorporated herein byreference. Nitrogen protecting groups include acyl, aryloyl, or carbamylgroups such as formyl, acetyl, propionyl, pivaloyl, t-butylacetyl,2-chloroacetyl, 2-bromoacetyl, trifluoroacetyl, trichloroacetyl,phthalyl, o-nitrophenoxyacetyl, α-chlorobutyryl, benzoyl,4-chlorobenzoyl, 4-bromobenzoyl, 4-nitrobenzoyl, and amino acids such asalanine, leucine, and phenylalanine; sulfonyl-containing groups such asbenzenesulfonyl, and p-toluenesulfonyl; carbamate forming groups such asbenzyloxycarbonyl, p-chlorobenzyloxycarbonyl,p-methoxybenzyloxycarbonyl, p-nitrobenzyloxycarbonyl,2-nitrobenzyloxycarbonyl, p-bromobenzyloxycarbonyl,3,4-dimethoxybenzyloxycarbonyl, 3,5-dimethoxybenzyloxycarbonyl,2,4-dimethoxybenzyloxycarbonyl, 4-methoxybenzyloxycarbonyl,2-nitro-4,5-dimethoxybenzyloxycarbonyl,3,4,5-trimethoxybenzyloxycarbonyl,1-(p-biphenylyl)-1-methylethoxycarbonyl,α,α-dimethyl-3,5-dimethoxybenzyloxycarbonyl, benzhydryloxy carbonyl,t-butyloxycarbonyl, diisopropylmethoxycarbonyl, isopropyloxycarbonyl,ethoxycarbonyl, methoxycarbonyl, allyloxycarbonyl,2,2,2,-trichloroethoxycarbonyl, phenoxycarbonyl, 4-nitrophenoxycarbonyl, fluorenyl-9-methoxycarbonyl, cyclopentyloxycarbonyl,adamantyloxycarbonyl, cyclohexyloxycarbonyl, and phenylthiocarbonyl,alkaryl groups such as benzyl, triphenylmethyl, and benzyloxymethyl, andsilyl groups, such as trimethylsilyl. Preferred nitrogen protectinggroups are alloc, formyl, acetyl, benzoyl, pivaloyl, t-butylacetyl,alanyl, phenylsulfonyl, benzyl, t-butyloxycarbonyl (Boc), andbenzyloxycarbonyl (Cbz).

By “oxo” is meant ═O.

By an “oxygen protecting group” is meant those groups intended toprotect an oxygen containing (e.g., phenol, hydroxyl, or carbonyl) groupagainst undesirable reactions during synthetic procedures. Commonly usedoxygen protecting groups are disclosed in Greene, “Protective Groups inOrganic Synthesis,” 3rd Edition (John Wiley & Sons, New York, 1999),which is incorporated herein by reference. Exemplary oxygen protectinggroups include acyl, aryloyl, or carbamyl groups, such as formyl,acetyl, propionyl, pivaloyl, t-butylacetyl, 2-chloroacetyl,2-bromoacetyl, trifluoroacetyl, trichloroacetyl, phthalyl,o-nitrophenoxyacetyl, α-chlorobutyryl, benzoyl, 4-chlorobenzoyl,4-bromobenzoyl, t-butyldimethylsilyl, tri-iso-propylsilyloxymethyl,4,4′-dimethoxytrityl, isobutyryl, phenoxyacetyl,4-isopropylpehenoxyacetyl, dimethylformamidino, and 4-nitrobenzoyl;alkylcarbonyl groups, such as acyl, acetyl, propionyl, and pivaloyl;optionally substituted arylcarbonyl groups, such as benzoyl; silylgroups, such as trimethylsilyl (TMS), tert-butyldimethylsilyl (TBDMS),tri-iso-propylsilyloxymethyl (TOM), and triisopropylsilyl (TIPS);ether-forming groups with the hydroxyl, such methyl, methoxymethyl,tetrahydropyranyl, benzyl, p-methoxybenzyl, and trityl; alkoxycarbonyls,such as methoxycarbonyl, ethoxycarbonyl, isopropoxycarbonyl,n-isopropoxycarbonyl, n-butyloxycarbonyl, isobutyloxycarbonyl,sec-butyloxycarbonyl, t-butyloxycarbonyl, 2-ethylhexyloxycarbonyl,cyclohexyloxycarbonyl, and methyloxycarbonyl; alkoxyalkoxycarbonylgroups, such as methoxymethoxycarbonyl, ethoxymethoxycarbonyl,2-methoxyethoxycarbonyl, 2-ethoxyethoxycarbonyl, 2-butoxyethoxycarbonyl,2-methoxyethoxymethoxycarbonyl, allyloxycarbonyl, propargyloxycarbonyl,2-butenoxycarbonyl, and 3-methyl-2-butenoxycarbonyl;haloalkoxycarbonyls, such as 2-chloroethoxycarbonyl,2-chloroethoxycarbonyl, and 2,2,2-trichloroethoxycarbonyl; optionallysubstituted arylalkoxycarbonyl groups, such as benzyloxycarbonyl,p-methylbenzyloxycarbonyl, p-methoxybenzyloxycarbonyl,p-nitrobenzyloxycarbonyl, 2,4-dinitrobenzyloxycarbonyl,3,5-dimethylbenzyloxycarbonyl, p-chlorobenzyloxycarbonyl,p-bromobenzyloxy-carbonyl, and fluorenylmethyloxycarbonyl; andoptionally substituted aryloxycarbonyl groups, such as phenoxycarbonyl,p-nitrophenoxycarbonyl, o-nitrophenoxycarbonyl,2,4-dinitrophenoxycarbonyl, p-methyl-phenoxycarbonyl,m-methylphenoxycarbonyl, o-bromophenoxycarbonyl,3,5-dimethylphenoxycarbonyl, p-chlorophenoxycarbonyl, and2-chloro-4-nitrophenoxy-carbonyl); substituted alkyl, aryl, and alkarylethers (e.g., trityl; methylthiomethyl; methoxymethyl; benzyloxymethyl;siloxymethyl; 2,2,2,-trichloroethoxymethyl; tetrahydropyranyl;tetrahydrofuranyl; ethoxyethyl; 1-[2-(trimethylsilyl)ethoxy]ethyl;2-trimethylsilylethyl; t-butyl ether; p-chlorophenyl, p-methoxyphenyl,p-nitrophenyl, benzyl, p-methoxybenzyl, and nitrobenzyl); silyl ethers(e.g., trimethylsilyl; triethylsilyl; triisopropylsilyl;dimethylisopropylsilyl; t-butyldimethylsilyl; t-butyldiphenylsilyl;tribenzylsilyl; triphenylsilyl; and diphenymethylsilyl); carbonates(e.g., methyl, methoxymethyl, 9-fluorenylmethyl; ethyl;2,2,2-trichloroethyl; 2-(trimethylsilyl)ethyl; vinyl, allyl,nitrophenyl; benzyl; methoxybenzyl; 3,4-dimethoxybenzyl; andnitrobenzyl); carbonyl-protecting groups (e.g., acetal and ketal groups,such as dimethyl acetal, and 1,3-dioxolane; acylal groups; and dithianegroups, such as 1,3-dithianes, and 1,3-dithiolane); carboxylicacid-protecting groups (e.g., ester groups, such as methyl ester, benzylester, t-butyl ester, and orthoesters; and oxazoline groups.

As noted, substituents may be optionally substituted with halo,optionally substituted C₃₋₁₀ carbocyclyl; optionally substituted C₁₋₉heterocyclyl having one to four heteroatoms independently selected fromO, N, and S; optionally substituted C₆₋₂₀ aryl; optionally substitutedC₁₋₉ heteroaryl having one to four heteroatoms independently selectedfrom O, N, and S; oxo; —CN; —NO₂; —OR_(a); —N(R_(a))₂; —C(═O)R_(a);—C(═O)OR_(a); —S(═O)₂R_(a); —S(═O)₂OR_(a); —P(═O)R_(a2);—O—P(═O)(OR_(a))₂, or —P(═O)(OR_(a))₂, or an ion thereof; wherein eachR_(a) is independently H, C₁₋₆ alkyl; optionally substituted C₃₋₁₀carbocyclyl; optionally substituted C₁₋₉ heterocyclyl having one to fourheteroatoms independently selected from O, N, and S; optionallysubstituted C₆₋₂₀ aryl; optionally substituted C₁₋₉ heteroaryl havingone to four heteroatoms independently selected from O, N, and S; whenbound to an oxygen atom, an oxygen protecting group; or when bound to anitrogen atom, a nitrogen protecting group. Cyclic substituents may alsobe substituted with C₁₋₆ alkyl.

Exemplary ions of substituent groups are as follows: an exemplary ion ofhydroxyl is —O⁻; an exemplary ion of —COOH is —COO⁻; exemplary ions of—PO₃H₂ are —PO₃H⁻ and —PO₃ ²⁻; an exemplary ion of —PO₃HR_(a) is—PO₃R_(a) ⁻, where R_(a) is not H; exemplary ions of —PO₄H₂ are —PO₄H⁻and —PO₄ ²⁻; and an exemplary ion of —SO₃H is —SO₃ ⁻.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 . ¹H NMR Spectrum of4,4′-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyric acid (2,6-DBEAQ). ¹HNMR (500 MHz, DMSO-d₆): δ 8.11 (d, J=8.6 Hz, 1H), 11.79 (s, 1H), 7.54(d, J=2.6 Hz, 1H), 7.38 (dd, J=8.7, 2.6 Hz, 1H), 4.20 (t, J=6.4 Hz, 2H),2.43 (t, J=7.3, 2H), 2.00 (m, 2H). Solvent peaks are the ones that arenot integrated. Final yield: 57%.

FIG. 2 . ¹H NMR spectrum of 1,8-DBEAQ. 1H NMR (500 MHz, DMSO-d₆): δ12.17 (s, 2H), 7.71 (m, 2H), 7.65 (m, 2H), 7.52 (dd, J=8.3, 1.3 Hz, 2H),4.15 (t, J=6.3 Hz, 4H), 2.53 (t, J=7.3 Hz, 4H), 1.89 (m, 4H). Finalyield: 64.4%.

FIG. 3 . ¹H NMR spectrum of 1,2-DBEAQ. 1H NMR (500 MHz, DMSO-d₆): δ12.15 (s, 2H), 8.11 (m, 2H), 7.97 (dd, J=8.6, 1.4 Hz, 1H), 7.85 (m, 2H),7.51 (d, J=8.7 Hz, 1H), 4.14 (t, J=6.4 Hz, 2H), 4.00 (t, J=6.2 Hz, 2H),2.51 (t, J=7.4 Hz, 2H), 2.43 (t, J=7.2 Hz, 2H), 2.01 (m, 4H). Finalyield: 72.1%.

FIGS. 4A-4B. (A) Rotating Disk Electrode study of the reduction of 5 mM2,6-DBEAQ in 1M KOH on a glassy carbon electrode at rotation ratesbetween 400 and 2500 rpm. (B) Levich plot (limiting current vs. squareroot of rotation rate in rad/s) of 5 mM 2,6-DBEAQ in 1 M KOH. Data aretaken from the current at −0.8 V vs. SHE. The slope yields a diffusioncoefficient for the oxidized form of 2,6-DBEAQ of 1.58×10⁻⁶ cm²/s.

FIGS. 5A-5B. (A) Cyclic voltammogram of 1,2-(blue), 2,6-(gray), and1,8-(green) DBEAQ. The redox potential versus SHE of each isomer is alsoindicated. Measurements were made at a scan rate of 25 mV s⁻¹. (B) CV of2,6-DBEAQ. Measurements were made at a scan rate of 10 mV s⁻¹ on apolished glassy carbon electrode. The dashed lines indicate thepotentials at the peaks in the current density. The arrows indicate theCV scan direction.

FIG. 6 . Capacitance-corrected CV of 5 mM 2,6-DBEAQ in 1 M KOH (solidgrey line). The dashed red line represents the simulated total currentarising from two successive one-electron reductions with reductionpotentials of −0.517 and −0.511 V vs. SHE, according to the procedureoutlined in a previous publication.¹ Each simulated reduction has a rateconstant k_(o)=7×10⁻³ cm/s and α=0.5. The diffusion coefficient of allredox states of 2,6-DBEAQ (oxidized, semiquinoid, and reduced) wasassumed to be 1.58×10⁻⁶ cm²/s.

FIG. 7 . 9-membered Square Scheme of AQ Reduction. The horizontaldirection indicates electron transfer onto the molecule at a givenpotential, i.e., E₁ and E₂, and the vertical direction indicatesprotonation of the species depending on the pK_(a) of the generatedanion and the solution pH. AQ states that are unstable under alkalineconditions are shown in grey. Blue arrows indicate PCET.

FIGS. 8A-8C. (A) Cell schematic for unbalanced compositionally symmetriccell cycling and full cell cycling. (B) Unbalanced compositionallysymmetric cell cycling of 0.1 M 2,6-DBEAQ showing capacity as a functionof time. Inset in the plot is the temporal capacity fade rate from alinear fit of the first 5 days of cycling. (C) Unbalancedcompositionally symmetric cell cycling of 0.65 M 2,6-DBEAQ capacity as afunction of time. Inset in the plot is the temporal capacity fade ratefrom a linear fit of the first 5 days of cycling. In both (B) and (C),the capacity-limiting side was 5 mL 2,6-DBEAQ while thenon-capacity-limiting side was 10 mL 2,6-DBEAQ, both at pH 14.Capacities were obtained by full potentiostatic reduction and oxidationat +/−0.2 V of the capacity-limiting side; the potential was switchedwhen the magnitude of the current density decayed to 2 mA/cm². Note thatthe y-axis scales in (B) and (C) represent about 2% of the capacity ofthe capacity-limiting side.

FIG. 9 . Unbalanced compositionally-symmetric cell cycling of 0.10 M2,6-DBEAQ and 2,6-DHAQ, showing capacity vs. time, both in 1 M KOH (pH14). The capacity-limiting side was 5 mL 0.1 M 2,6-DBEAQ or DHAQ, whilethe non-capacity-limiting side was 10 mL of the same. Capacities wereobtained by full potentiostatic reduction and oxidation of 5 mL ofcapacity-limiting side; potential was switched between ±0.2 V whenmagnitude of current dropped to 2 mA/cm². Cell cycle period is ˜20 min.

FIGS. 10A-10C. NMR evidence of decomposition of 2,6-DBEAQ in theoxidized form. ¹H NMR spectra (500 MHz, 1 M KOD in D₂O with 10 mMNaCH₃SO₃ internal standard) of (A) the oxidized form of 2,6-DBEAQ in pH14 aqueous solution (1 M KOH); (B) 2,6-DBEAQ treated at 75° C. for 8days at 0.1 M concentration in pH 14 aqueous solution (1 M KOH) in thereduced form and then reoxidized in order to compare to samples testedin the oxidized form; (C) 2,6-DBEAQ treated at 75° C. for 8 days at 0.1M concentration in pH 14 aqueous solution (1 M KOH) in the oxidizedform.

FIGS. 11A-11D. Identification of a decomposition product of 2,6-DBEAQ.1H NMR spectra (500 MHz, 1 M KOD in D₂O with 10 mM NaCH₃SO₃ internalstandard) of (A) aliphatic region of the oxidized form of 2,6-DBEAQ inpH 14 aqueous solution (1 M KOH); (B) γ-hydroxybutyrate; (C) aliphaticregion of 2,6-DBEAQ treated at 75° C. for 8 days at 0.1 M concentrationin pH 14 aqueous solution (1 M KOH) in the oxidized form; (D) aliphaticregion of 2,6-DBEAQ treated at 75° C. for 8 days at 0.1 M concentrationin pH 14 aqueous solution (1 M KOH) in the oxidized form with 10 mMy-hydroxybutyrate added to the NMR sample.

FIG. 12 . Time course of 2,6-DBEAQ decomposition at various temperature,pH, and concentration conditions. The remaining concentration of2,6-DBEAQ relative to the initial concentration vs. time (in days) forsamples treated at 65° C. and 95° C., at pH 12 and pH 14, and at 0.1 Mand 0.5 M concentration in the oxidized form.

FIG. 13A-13C. Evidence of pseudo-first order decomposition kinetics withrespect to 2,6-DBEAQ. The remaining concentration of 2,6-DBEAQ relativeto the initial concentration vs. time (in days) for samples treated at95° C., at pH 14, and at 0.1 M concentration in the oxidized form,plotted (A) on a semi-logarithmic scale; (B) on a linear scale; (C) asthe reciprocal of the 2,6-DBEAQ concentration relative to the initialconcentration vs. time.

FIG. 14A-14B. (A) The permeability of the oxidized form of 2,6-DBEAQ,2,6-DHAQ, and potassium ferricyanide across a FUMASEP® E-610 (K)membrane. (B) 2,6-DHAQ in Nafion 212 and FUMASEP® E-610 (K). The sloperepresents the permeability.

FIG. 15 . Capacity and current efficiency (capacity on discharge dividedby the capacity of the preceding charge step) as functions of timeduring galvanostatic cycling of a cell with a negolyte of 5.5 mL 0.1 M2,6-DBEAQ and posolyte of 15 mL 0.20 M potassium ferrocyanide and 0.08 Mpotassium ferricyanide, both at pH 14 (880 cycles with each cycle 6.5minutes long). Galvanostatic cycling was performed at 100 mA/cm² withvoltage cutoffs of 1.4 and 0.6 V on charge and discharge, respectively.

FIG. 16A-16B. Unbalanced compositionally-symmetric cell cycling of 0.10M (A) 1,2- and (B) 1,8-DBEAQ, showing capacity vs. time, both in 0.01 MKOH (pH 12). The capacity-limiting side was 5 mL 0.1 M DBEAQ, while thenon-capacity-limiting side was 10 mL of the same. Capacities wereobtained by full potentiostatic reduction and oxidation of 5 mL ofcapacity-limiting side; potential was switched between ±0.2 V whenmagnitude of current dropped to 2 mA/cm². Linear fits between 0.01 and10%/day are drawn to compare against experimental data.

FIG. 17 . Unbalanced compositionally-symmetric cell cycling of 0.10 M2,6-DBEAQ, showing capacity vs. time in 0.01 M KOH (pH 12). Thecapacity-limiting side was 5 mL 0.1 M 2,6-DBEAQ, while thenon-capacity-limiting side was 10 mL of the same. Capacities wereobtained by full potentiostatic reduction and oxidation of 5 mL ofcapacity-limiting side; potential was switched between ±0.2 V whenmagnitude of current dropped to 2 mA/cm². Dashed lines with slopes of0.01 and 0.1%/day are drawn to compare against experimental data, whichshowed a temporal fade rate of approximately 0.007%/day.

FIGS. 18A-18B. (A) Pourbaix diagram of 2,6-DBEAQ with a slope fit to thedata of −35 mV/pH below pH 12. Above pH ˜11.5, the potential ispH-independent, indicating that both oxidized and reduced forms of2,6-DBEAQ are deprotonated. The dashed line has zero slope. (b)Capacitance-corrected CVs of 5 mM 2,6-DBEAQ in 1 M KOH (black curve) andpH 12 buffer (red curve) solution. We hypothesize the wider peakseparation in the buffered pH 12 solution to be a result of slowerkinetics for the first and second electron reduction steps' rather thana larger difference between their redox potentials.

FIGS. 19A-19D. (A) Cell voltage versus discharge current density at roomtemperature at 10%, 30%, 50%, 70%, 90%, and 100% SOC. Electrolytesincluded 5 mL of 2,6-DBEAQ (negolyte) at pH 12 (10 mM KOH) and 38 mL of0.3 M potassium ferrocyanide and 0.1 M potassium ferricyanide (posolyte)at pH 12. (B) OCV, high-frequency, and polarization ASR versus SOC. (C)Galvanostatic charge and discharge curves from 25 to 250 mA/cm². Thevertical dashed lines indicate the maximum capacity realized withpotentiostatic charge and discharge at the voltage cutoffs (1.4 and 0.6V, respectively), as well as the theoretical capacity. (D) Columbicefficiency, round-trip energy efficiency, and capacity utilization as apercentage of potentiostatic capacity versus current density.

FIG. 20 . Power density versus current density for 0.5 M 2,6-DBEAQ at20° C. at 50% and ˜100% SOC.

FIGS. 21A-21B. (A) Current efficiency (squares) and charge (upwardpointing triangles) and discharge (downward pointing triangles) capacityversus time and cycle number for a negolyte-limited 2,6-DBEAQ-Fe(CN)₆cell. The cell was cycled galvanostatically at 100 mA/cm² between 1.4and 0.6 V, and each half-cycle ended with a potentiostatic hold untilthe magnitude of the current density fell below 2 mA/cm². The negolyteincluded 5 mL of 0.5 M 2,6-DBEAQ at pH 12 while the posolyte included 30mL of 0.3 M potassium ferrocyanide and 0.1 M potassium ferricyanide atpH 12. (B) Evolution of charge (upward pointing triangles) and discharge(downward pointing triangles) capacity for extended charge cycling at100 mA/cm². After every twentieth galvanostatic cycle, a potentiostaticcharge-discharge cycle was performed, the discharge capacity of which isshown in the open circles. Because the concentration of 2,6-DBEAQ is atits maximum after each potentiostatic discharge, the capacity of thegalvanostatic charge step immediately following potentiostaticcharge-discharge is higher than in subsequent cycles. This particularcell showed only 65% of its theoretical capacity because a largefraction of salt was left over from the synthesis of 2,6-DBEAQ.

FIGS. 22A-22B. (A) NMR analysis of electrolytes from a cellcompositionally similar to that in FIG. 19 and cycled for 11 days. Theorigin of the peak shifts in NMR is unknown but could stem fromdifferences slight differences in pH. (B) CV measurement of cycledposolyte showing no 2,6-DBEAQ redox peaks within the potential window(vs. Ag/AgCl).

FIGS. 23A-23F. Comparison of the stability of DPPEAQ with respect toalkyl chain cleavage at pH 9 and 12 to that of 2,6-DBEAQ at pH 12. ¹HNMR spectra (500 MHz, 10 mM NaCH₃SO₃ internal standard, δ 2.6 ppm) of(A) DPPEAQ at pH 9; (B) DPPEAQ at pH 9, treated for 6 days at 95° C. and0.1 M concentration; (C DPPEAQ at pH 12; (D) DPPEAQ at pH 12, treatedfor 6 days at 95° C. and 0.1 M concentration; (E) 2,6-DBEAQ at pH 12;(F) 2,6-DBEAQ at pH 12, treated for 6 days at 95° C. and 0.1 Mconcentration. After treatment, 2,6-DBEAQ at pH 12 exhibits 15%decomposition, whereas DPPEAQ at both pH 9 and pH 12 exhibits nosignificant decomposition.

FIGS. 24A-24D. Comparison of the stability of DPPEAQ with respect toalkyl chain cleavage to that of 2,6-DBEAQ at pH 14. ¹H NMR spectra (500MHz, 10 mM NaCH₃SO₃ internal standard, δ 2.6 ppm) of (A) DPPEAQ at pH14; (B) DPPEAQ at pH 14, treated for 5.5 days at 95° C. and 0.1 Mconcentration; (C) 2,6-DBEAQ at pH 14; (D) 2,6-DBEAQ at pH 14, treatedfor 5.5 days at 95° C. and 0.1 M concentration. After treatment,2,6-DBEAQ and DPPEAQ both exhibit 45% decomposition. The relativeintegrals and splitting of the peaks in the spectra of DPPEAQ aftertreatment are consistent with the products of(3-hydroxypropyl)phosphonate cleavage. Samples of DPPEAQ at 0.1 Mconcentration and pH 14 in both the oxidized and reduced form werestored in FEP bottles and heated in an oven at 65° C. for 2 weeks. Theextent of decomposition was determined by ¹H NMR, with peak integralsmeasured relative to an internal standard of NaCH₃SO₃ prepared at 10 mMconcentration in D₂O.

FIG. 25 . ¹H NMR spectrum of DPPEAQ. Solvent peaks are those that arenot integrated.

FIGS. 26A-26C. Comparison of DPPEAQ stability in the oxidized andreduced forms at pH 14. ¹H NMR spectra (500 MHz, 10 mM NaCH₃SO₃ internalstandard, δ 2.6 ppm) of (A) DPPEAQ at pH 14; (B) DPPEAQ at pH 14,treated for 2 weeks at 65° C. and 0.1 M concentration in the oxidizedform; (C) DPPEAQ at pH 14, treated for 12 days at 65° C. and 0.1 Mconcentration in the reduced form (sample reoxidized prior to collecting¹H NMR spectrum). Unlike the oxidized form of DPPEAQ, the reduced formexhibits robust chemical stability after treatment, even at pH 14.

FIGS. 27A-27B. (A) Rotating Disk Electrode study of the reduction of 5mM DPPEAQ in 1M KOH on a glassy carbon electrode at rotation ratesbetween 400 and 2000 rpm. (B) Levich plot (limiting current vs. squareroot of rotation rate in rad/s) of 5 mM DPPEAQ in 1 M KOH. The slopeyields a diffusion coefficient for the oxidized form of DPPEAQ of1.37×10⁻⁶ cm²/s.

FIGS. 28A-28D. (A) Cyclic voltammograms of 1 mM DPPEAQ at pH 9 buffer(black solid), pH 12 buffer (red solid), pH 9 unbuffered (black dash)and pH 12 unbuffered (red dash), at a scan rate of 100 mV s⁻¹, on aglassy carbon working electrode. (B) Galvanostatic cycling of the DPPEAQcell at 20 mA/cm² for 16 consecutive cycles in the glove bag.Electrolytes included 6.5 mL 0.1 M DPPEAQ (negolyte) in 1 M KCl solutionat pH 9 and 40 mL 0.1 M potassium ferrocyanide and 0.01 M potassiumferricyanide (posolyte) in 1 M KCl solution at pH 9. The pH probe wasimmersed in the negolyte to monitor the pH of the solution.Charge/discharge capacity, current efficiency (CE), and pH of thenegolyte before and after charging were plotted as functions of thecycle number. (C) Representative curves of cell potential and negolytepH vs. time. (D) Repeating experiments (B) and (C) but in a glove boxinstead of in the glove bag.

FIG. 29 . Pourbaix diagram of DPPEAQ with a slope of −30 mV/pH fit tothe data below pH 12. Above pH ˜12.4, the potential is pH-independent,indicating that both oxidized and reduced forms of DPPEAQ aredeprotonated. The dashed line has zero slope.

FIGS. 30A-30D. (A) Unbalanced compositionally-symmetric cell cycling of0.1 M DPPEAQ at pH 13, showing capacity as a function of time.Capacities were obtained by full potentiostatic reduction and oxidationat ±0.2 V of the capacity-limiting side. (B) Full cell OCV vs. SOC atroom temperature at 10, 50, and 90% SOC. Electrolytes included 5 mL 0.5M DPPEAQ (negolyte) at pH 9 and 80 mL 0.4 M potassium ferrocyanide and0.1 M potassium ferricyanide (posolyte) at pH 9. (C) Cell potential andpower density vs. current density. (D) Current efficiency (squares) andcharge (upward-pointing triangles) and discharge (downward-pointingtriangles) capacity vs. time and cycle number. The cell was cycledgalvanostatically at 100 mA/cm² between 1.5 and 0.5 V, and eachhalf-cycle ended with a potentiostatic hold until the magnitude of thecurrent density fell below 2 mA/cm². Inset: capacity vs. cell voltage atthe 1^(st), the 10^(th), the 100^(th) and the 480^(th) cycle,respectively.

FIG. 31 . Representative curves of cell potential and negolyte pH versustime of the DPPEAQ cell in the glove box. Electrolytes included 5 mL 0.5M DPPEAQ (negolyte) at pH 9 and 80 mL 0.4 M potassium ferrocyanide and0.1 M potassium ferricyanide (posolyte) at pH 9. The pH probe wasimmersed in the negolyte to monitor the pH of the solution.

DETAILED DESCRIPTION

We disclose quinone compounds and related species that possesssignificant advantages when used as a redox active material in abattery, e.g., a redox flow battery. In particular, the compoundsprovide RFBs with extremely high capacity retention. For example, RFBsof the invention can be cycled for 500 times with negligible loss ofcapacity, and such batteries could be employed for years of service.Thus, the invention provides a high efficiency, long cycle life redoxflow battery with reasonable power cost, low energy cost, and all theenergy scaling advantages of a flow battery. Quinone-to-hydroquinonecycling occurs rapidly and reversibly and provides high current density(high current density is very important because the cost per kW of thesystem is typically dominated by the electrochemical stack's cost perkW, which is inversely proportional to the power density—the product ofcurrent density and voltage), high efficiency, and long lifetime in aflow battery.

Batteries of the invention employ a first redox active material and asecond redox active material, which is the quinone compound of theinvention. Typically, the quinone compound is in the negolyte andtherefore in its reduced, hydroquione form when fully charged. As thebattery is discharged, the hydroquinone is then oxidized to itscorresponding quinone. During charging the quinone is similarly reducedto the hydroquinone. In the batteries of the invention, energy is storedin the quinone/hydroquinone, which is not merely a charge transfercatalyst in the reduction or oxidation of a secondary species.

Exemplary quinone compounds are of formula I:

wherein each of R₁-R₈ is independently H; halo; optionally substitutedC₁₋₆ alkyl; optionally substituted C₃₋₁₀ carbocyclyl; optionallysubstituted C₁₋₉ heterocyclyl having one to four heteroatomsindependently selected from O, N, and S; optionally substituted C₆₋₂₀aryl; optionally substituted C₁₋₉ heteroaryl having one to fourheteroatoms independently selected from O, N, and S; oxo, —NO₂; —OR_(a);—N(R_(a))₂; —C(═O)R_(a); —C(═O)OR_(a); —S(═O)₂R_(a); —S(═O)₂OR_(a);—OS(═O)₂OR_(a); —P(═O)R_(a2); —P(═O)(OR_(a))₂; and —OP(═O)(OR_(a))₂;—X₁- L₁-C(O)O—Y₁; —X₂-L₂-C(O)O—Y₂; —X₃-L₃-P(═O)(OY₃)₂; or—X₄-L₄-P(═O)(OY₄)₂;wherein each R_(a) is independently H; optionally substituted C₁-6alkyl; optionally substituted C₃₋₁₀ carbocyclyl; optionally substitutedC₁₋₉ heterocyclyl having one to four heteroatoms independently selectedfrom O, N, and S; optionally substituted C₆₋₂₀ aryl; or optionallysubstituted C₁₋₉ heteroaryl having one to four heteroatoms independentlyselected from O, N, and S;X₁, X₂, X₃, and X₄ are independently O, S, or CH₂; L₁, L₂, L₃, and L₄are independently C₁-C₆ alkylene; and Y₁, Y₂, Y₃, and Y₄ areindependently H or optionally substituted C₁-C₆ alkyl, provided that oneand only one of R₁-R₈ is —X₁-L₁-C(O)O—Y, or —X₃-L₃-P(═O)(OY₃)₂ and oneand only one of R₁-R₈ is —X₂-L₂-C(O)O—Y₂ or —X₄-L₄-P(═O)(OY₄)₂. Inparticular embodiments, each of R₁-R₈ is independently H; halo;optionally substituted C₁₋₆ alkyl; optionally substituted C₃₋₁₀carbocyclyl; optionally substituted C₁₋₉ heterocyclyl having one to fourheteroatoms independently selected from O, N, and S; optionallysubstituted C₆₋₂₀ aryl; optionally substituted C₁₋₉ heteroaryl havingone to four heteroatoms independently selected from O, N, and S; oxo,—NO₂; —OR_(a); —N(R_(a))₂; —C(═O)R_(a); —C(═O)OR_(a); —S(═O)₂R_(a);—S(═O)₂₀R_(a); —P(═O)R_(a2); and —P(═O)(OR_(a))₂—X₁-L₁-C(O)O—Y₁;—X₂-L₂-C(O)O—Y₂; —X₃-L₃-P(═O)(OY₃)₂; or —X₄-L₄-P(═O)(OY₄)₂, e.g., whenthe pH of the electrolyte is ≥7 e.g., the pH is from 8-13. Exemplarycompounds are 2,6-DBEAQ, 1,2-DBEAQ, 1,8-DBEAQ, and DPPEAQ.

Compounds also include salts, ions (e.g., —COO⁻, —PO₃H⁻, or —PO₃ ²⁻), ora hydroquinone thereof. As will be understood, a hydroquinone of acompound of formula I has the formula:

Salts or ions may also be in hydroquinone form.

Examples of first redox active materials that may be used in a batteryof the invention are bromine, chlorine, iodine, oxygen, vanadium,chromium, cobalt, iron (e.g., ferricyanide/ferrocyanide or a ferrocenederivative, e.g., as described in PCT/US2017/046783), aluminum, e.g.,aluminum(III) biscitrate monocatecholate, manganese, cobalt, nickel,copper, or lead, e.g., a manganese oxide, a cobalt oxide, or a leadoxide. Other redox active species suitable for use in batteries of theinvention are described in WO 2014/052682, WO 2015/048550, and WO2016/144909, the redox active species of which are incorporated byreference.

Electrolytes

In some embodiments, the electrolytes are both aqueous, where the firstand second redox active species are in aqueous solution or aqueoussuspension. In addition to the redox active species, the electrolyte mayinclude other solutes, e.g., acids (e.g., HCl) or bases (e.g., NaOH orKOH) or alcohols (e.g., methyl, ethyl, or propyl) and other co-solventsto increase the solubility of a particular quinone/hydroquinone. Incertain embodiments, the pH of the electrolyte may be >7, e.g., at least8, 9, 10, 11, 12, 13, or 14, 8-14, 9-14, 10-14, 11-14, 12-14, 13-14, orabout 14. The electrolyte may or may not be buffered to maintain aspecified pH. The first and second redox actives species will be presentin amounts suitable to operate the battery, for example, from 0.1-15 M,or from 0.1-10 M. In some embodiments, the solution is at least 10%,20%, 30%, 40%, 50%, 60%, 70%, or 80% water, by mass. Quinones,hydroquinones, salts, and/or ions thereof may be present in a mixture.

Electrode Materials

Electrodes for use with an organic compound or ion thereof (e.g.,quinone or hydroquinone) include any carbon electrode, e.g., glassycarbon electrodes, carbon paper electrodes, carbon felt electrodes, orcarbon nanotube electrodes. Titanium electrodes may also be employed.Electrodes can also be made of a high specific surface area conductingmaterial, such as a nanoporous metal sponge (T. Wada, A. D. Setyawan, K.Yubuta, and H. Kato, Scripta Materialia 65, 532 (2011)), which has beensynthesized previously by electrochemical dealloying (J. D. Erlebacher,M. J. Aziz, A. Karma, N. Dmitrov, and K. Sieradzki, Nature 410, 450(2001)), or a conducting metal oxide, which has been synthesized by wetchemical methods (B. T. Huskinson, J. S. Rugolo, S. K. Mondal, and M. J.Aziz, arXiv:1206.2883 [cond-mat.mtrl-sci]; Energy & EnvironmentalScience 5, 8690 (2012); S. K. Mondal, J. S. Rugolo, and M. J. Aziz,Mater. Res. Soc. Symp. Proc. 1311, GG10.9 (2010)). Chemical vapordeposition can be used for conformal coatings of complex 3D electrodegeometries by ultra-thin electrocatalyst or protective films. Electrodessuitable for other redox active species are known in the art.

Ion Conducting Barriers

The ion conducting barrier allows the passage of ions, such as sodium orpotassium, but not a significant amount of the quinone, hydroquinone, orother redox active species. Examples of ion conducting barriers areNAFION®, i.e., sulfonated tetrafluoroethylene basedfluoropolymer-copolymer, FUMASEP®, i.e., non-fluorinated, sulfonatedpolyaryletherketone-copolymer, e.g., FUMASEP® E-620(K), hydrocarbons,e.g., polyethylene, and size exclusion barriers, e.g., ultrafiltrationor dialysis membranes with a molecular weight cut off of 100, 250, 500,or 1,000 Da. For size exclusion membranes, the required molecular weightcut off is determined based on the molecular weight of the organiccompound (e.g., quinone or hydroquinone) or other redox active speciesemployed. Porous physical barriers may also be included, when thepassage of redox active species is tolerable.

Additional Components

A battery of the invention may include additional components as is knownin the art. Quinones, hydroquinones, and other redox active species inaqueous solution or aqueous suspension are housed in a suitablereservoir. A battery may further include one or more pumps to pumpaqueous solutions or suspensions past one or both electrodes.Alternatively, the electrodes may be placed in a reservoir that isstirred or in which the solution or suspension is recirculated by anyother method, e.g., convection, sonication, etc. Batteries may alsoinclude graphite flow plates and corrosion-resistant metal currentcollectors.

The balance of the system around the cell includes fluid handling andstorage, and voltage and round-trip energy efficiency measurements canbe made. Systems configured for measurement of catholyte and anolyte(e.g., negolyte and posolyte) flows and pH, pressure, temperature,current density and cell voltage may be included and used to evaluatecells. Testing is performed as quinone and pH and the cell temperatureare varied. In one series of tests, the current density is measured atwhich the voltage efficiency drops to 90%. In another, the round-tripefficiency is evaluated by charging and discharging the same number ofamp-minutes while tracking the voltage in order to determine the energyconversion efficiency. This is done initially at low current density,and the current density is then systematically increased until theround-trip efficiency drops below 80%. Fluid sample ports can beprovided to permit sampling of both electrolytes, which will allow forthe evaluation of parasitic losses due to reactant crossover or sidereactions. Electrolytes can be sampled and analyzed with standardtechniques.

Suitable cells, electrodes, membranes, and pumps for redox flowbatteries are known in the art, e.g., WO 2014/052682, WO 2015/048550,and WO 2016/144909, the battery components of which are herebyincorporated by reference.

Example 1. Synthesis of 2,6-DBEAQ

2,6-di hydroxyanthraquinone (2,6-DHAQ) was purchased from AK Scientific.Methyl 4-bromobutyrate was purchased from VWR. All other chemicals werepurchased from Sigma Aldrich. All chemicals were used as received unlessspecified otherwise.

Dimethyl 4,4′-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate (1)2,6-DHAQ was first converted to its dipotassium salt (2,6-DHAQK₂) byadding 2,6-DHAQ (5 g, 20.8 mmol) to a beaker of absolute ethanol (200mL). Under vigorous stirring, potassium ethoxide (24 wt % solution inethanol, 32.6 mL, 83.3 mmol) was added dropwise. The mixture was stirredat room temperature for 15 minutes, and the red solid was vacuumfiltered, washed twice with cold absolute ethanol (20+20 mL), and driedunder vacuum overnight at 40° C. For the O-alkylation reaction,2,6-DHAQK₂ (2 g, 6.3 mmol) was added to a 100 mL round-bottom flaskfollowed by the addition of anhydrous DMF (40 mL) and anhydrous K₂CO₃(0.87 g, 6.3 mmol). The solution was first stirred and heated to 120° C.under nitrogen for 30 minutes, and then methyl 4-bromobutyrate (2 mL,15.8 mmol) was added. The reaction mixture was then brought to refluxfor 12 hours. After cooling to room temperature, DI water (25 mL) wasadded to the mixture to dissolve inorganic salt and to precipitate theproduct 1. The precipitate was vacuum filtered and washed thoroughlywith DI water (50 mL). The product was analyzed by ¹H NMR and used forthe next step reaction without further purification. Final yield: 57%.

4,4′-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyric acid, 2,6-DBEAQ (2)Product 1 (1 g, 2.23 mmol) was added along with KOH (0.52 g, 9 mmol) toa flask filled with water-methanol mixture (2:1 v/v, 60 mL). Thesolution was vigorously stirred and heated to 40° C. for 12 hours.During the reaction, all solids dissolved, and the solution turned intoa dark red color. After reaction, the solution was transferred to alarger 500 mL flask, diluted with DI water (200 mL), and glacial aceticacid was added until the solution pH dropped down to 4. The mixture wasvigorously stirred for 1 hour followed by vacuum filtration and thoroughwashing with DI water (100 mL). The product was dried under vacuum at40° C., analyzed by ¹H NMR and used for electrochemical measurementwithout further purification. Final yield: 99%.

The ¹H NMR spectrum of4,4′-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyric acid (2,6-DBEAQ) isshown in FIG. 1 .

2,6-di hydroxyanthraquinone (2,6-DHAQ) was purchased from AK Scientific.Methyl 4-bromobutyrate was purchased from VWR. All other chemicals werepurchased from Sigma Aldrich. All chemicals were used as received unlessspecified otherwise.

Dimethyl 4,4′-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate (1)2,6-DHAQ was first converted to its dipotassium salt (2,6-DHAQK₂) byadding 2,6-DHAQ (5 g, 20.8 mmol) to a 250 mL oven-dried flask ofdimethylformamide (250 mL). Under vigorous stirring, potassium ethoxide(6.1 g, 72.9 mmol) was added. The mixture solution was stirred at roomtemperature for 15 minutes. For the O-alkylation reaction, 2,6-DHAQK₂(6.5 g, 20.8 mmol) was mixed with anhydrous K₂CO₃ (14.3 g, 104 mmol) and4-bromobutyrate (12.4 mL, 104 mmol). The reaction mixture was thenheated to 95° C. overnight. After cooling to 0° C., DI water (150 mL)was added to the mixture to dissolve inorganic salt and to precipitatethe ester precursor of DBEAQ. The precipitate was vacuum filtered andwashed thoroughly with DI water (50 mL). The product was analyzed by ¹HNMR and used for the next step reaction without further purification.Final yield: 87%.

4,4′-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyric acid (2) The esterprecursor of DBEAQ (1 g, 2.23 mmol) was added along with KOH (0.52 g, 9mmol) to a flask filled with water-isopropanol mixture (2:1 v/v, 60 mL).The solution was vigorously stirred and heated to 60° C. for 12 hours.During the reaction, all solid dissolved, and the solution turned into adark red color. After reaction, the solution was transferred to a larger500 mL flask and diluted with DI water (200 mL). Glacial acetic acid wasadded until the solution pH dropped to 4. The mixture was vigorouslystirred for 1 hour, followed by vacuum filtration and thorough washingwith DI water (100 mL). The product was vacuum dried, analyzed by 1HNMR, and used for electrochemical measurement without furtherpurification. Final yield: 99%

The ¹H NMR spectrum of 2,6-DBEAQ is shown in FIG. 1 . 1,8- and1,2-isomers were synthesized similarly, using 1,2-DHAQ and 1,8-DHAQ asprecursors. Potassium tert-butoxide (8.16 g, 72.9 mmol) replacedpotassium ethoxide to deprotonate the 1,2-DHAQ and 1,8-DHAQ isomersbecause of the higher pK_(a) of the hydroxyl groups of these isomers. ¹HNMR spectra of the final products are shown in FIG. 2 and FIG. 3 ,respectively, along with their structures.

Solubility limits of all DBEAQ isomers were measured in their oxidizedforms by adding the potassium salt of DBEAQ (prepared by reacting DBEAQwith potassium hydroxide in water) until no further solid could bedissolved. After filtering the mixture through a PTFE 0.45 μm syringefilter, a saturated solution of DBEAQ in KOH was obtained. The saturatedsolution was then diluted by a known amount, and the concentration wasevaluated by UV-Vis (Ocean Optics Flame-S Spectrometer Assembly) at 280and 364 nm. The concentration was calculated according to apre-calibrated absorbance-concentration curve of known concentrations ofDBEAQ. Solubility of the oxygen-sensitive reduced form of DBEAQ was notmeasured but was assumed to be higher than the oxidized form because:(1) no precipitation after full electrochemical reduction of DBEAQ wasobserved and (2) increasing the number of negative charges from two forDBEAQ in the oxidized form to four for DBEAQ in the reduced form isexpected to make quinone-quinone interactions even more unfavorable andincrease its solubility.²

After the functionalization, the resulting DBEAQ isomers showed asignificant improvement of solubility versus their hydroxyl counterpartsat both pH 14 and pH 12. For instance, the room temperature solubilityof 2,6-DBEAQ exceeded 1 M in pH 14 and 0.5 M in pH 12 KOH solution(Table 1), as opposed to 0.6 M³ and 0.1 M for 2,6-DHAQ under similarconditions, respectively. Several avenues exist for further increasingreactant solubility and the corresponding energy density, such as theuse of mixed cations in the electrolytes⁴ or mixtures of different DBEAQisomers or different molecules with nearly the same reduction potential.

TABLE 1 Effect of pH and counter-ion on maximum solubilities ofpotassium ferrocyanide, potassium ferricyanide, and 2,6-DBEAQ in KOH.Ranges represent uncertainties in solubility based on different peaksused in UV-Vis calibration measurements. Compound Conditions MaximumSolubility (M) Potassium Ferrocyanide KOH, pH 14 0.5 KOH, pH 12 1.2-1.3Potassium Ferricyanide KOH, pH 14 1.05-1.13 KOH, pH 12 1.8-2.0 2,6-DBEAQKOH, pH 14 1.10 KOH, pH 12 0.60

Example 2. Cyclic Voltammetry (CV) of 2,6-DBEAQ

Glassy carbon was used as the working electrode for all three-electrodeCV tests. RDE experiments were conducted using a Pine InstrumentsModulated Speed Rotator AFMSRCE equipped with a 5 mm diameter glassycarbon working electrode, a Ag/AgCl reference electrode (BASi,pre-soaked in 3 M NaCl solution), and a graphite counter electrode. Theelectrode was rotated at a specific speed while the voltage was linearlyswept from −0.4 to −0.11 V vs. Ag/AgCl (FIG. 4A). The diffusioncoefficient of the oxidized form of 2,6-DBEAQ was calculated using theLevich equation, which relates the mass-transport-limited current to thenumber of electrons transferred (n), the area of the electrode (A), andthe concentration of redox-active species in the electrolyte (C), byplotting the mass-transport-limited current against the square root ofthe rotation rate (FIG. 4B) with the following parameters: n=2, F=96,485Coulombs/mol, A=0.196 cm², C=5 mM, kinematic viscosity of 1 M KOHv=1.08×10⁻⁶ m²/s. The resulting value of the diffusion coefficient forthe oxidized form of 2,6-DBEAQ is 1.58×10⁻⁶ cm²/s.

A first CV experiment was performed in which 2,6-DBEAQ was dissolved inpH 14 KOH solution at a concentration of 5 mM. CV measurements wereplotted in FIG. 5A and include results for the isomers of 2,6-DBEAQ. AllDBEAQ isomers showed very similar redox potential between −510 and −540mV vs. SHE, which would yield battery voltages above 1 V versusferri-/ferrocyanide posolyte. Despite the increase in reductionpotential compared with 2,6-DHAQ (−680 mV vs. SHE), their CVs exhibitedhigh reversibility with redox peaks ˜40 mV apart, much smaller than the˜90 mV value from 2,6-DHAQ.³ A Levich analysis was used to obtain thediffusion coefficient of the oxidized form of 2,6-DBEAQ (1.58×10⁻⁶cm²/s, FIGS. 4A-4B), which was then used in a CV simulation of its redoxkinetics (FIG. 6 ). The results show, in agreement with an analogousstudy of the 2,6-DHAQ CV, that the peak shape and separation isconsistent with two one-electron reduction steps at differentpotentials, E₁ and E₂, whose values are modulated by the energetics ofsemiquinone reduction (FIG. 7 ). In comparison with 2,6-DHAQ, whichexhibited an E₁-E₂ separation of 60 mV³, the 2,6-DBEAQ CV exhibits amuch smaller E₁-E₂ separation of 6 mV, implying that 2,6-DHAQ is morethermodynamically susceptible to the formation of semiquinone radicalsthan is 2,6-DBEAQ.

A second CV experiment was performed in which 2,6-DBEAQ was dissolved inpH 14 KOH solution at a concentration of 2 mM. CV measurements wereplotted in FIG. 4C. The standard equilibrium potential was determined tobe −0.49 V vs. SHE. The separation between the peaks in the CV curveswas 42 mV, indicating fairly rapid and reversible electrochemicalreactions at the surface of the electrodes.

Example 3. Theoretical Calculations of DBEAQ Isomer Susceptibility toAlkyl Chain Cleavage

The stability of disubstituted DBEAQ isomers against the loss of thealkyl groups was evaluated computationally. The first approach taken wasto evaluate the relative reaction energies of the loss of the alkylchain of both the oxidized and reduced forms of different isomers ofDBEAQ. These reactions are shown in Scheme 3.

Optimizations were conducted at the PM7 level of theory with the COSMOsolvation model using the MOPAC package.⁵ Up to 20 conformers for eachmolecule were generated using RDKit.⁶ Because the molecule is inalkaline conditions, the energies were evaluated for the deprotonatedforms where explicit potassium cations are placed near the negativecharges. The relative energies are shown in Table 2. Two importantobservations from these results are: (1) the reaction energiesassociated with chain loss are more thermodynamically favorable for theoxidized form in all cases, except in 2,6-DBEAQ, where the reduced formis slightly more susceptible. Second, the thermodynamic stability ofboth the oxidized and reduced forms of 2,6-DBEAQ are among the highestof all the isomers examined.

TABLE 2 Relative Energies of DBEAQ. Thermodynamic DBEAQ Loss of Chain(eV) Reaction Energy to form Chain Oxidized Reduced TetrahedralIntermediate (eV) Positions Form^(a,b) Form^(a,b) From OxidizedForm^(a,c) 2,6 0.00 −0.06 0.00 1,2 −0.38 −0.19 −0.21 1,3 −0.27 −0.09−0.38 1,4 −0.33 0.03 −0.20 1,5 −0.32 −0.13 −0.35 1,6 −0.35 0.00 −0.291,7 −0.56 −0.21 −0.21 1,8 −0.34 0.02 −0.27 2,3 −0.30 0.71 −0.21 2,7 0.000.86 −0.04 ^(a)Energies calculated at the PM7 level of theory with theCOSMO solvation model; ^(b)Reaction energies are tabulated relative tothe energy of 2,6-DBEAQ losing an alkyl chain in its oxidized form;^(c)Reaction energies to form tetrahedral intermediate are relative tothe reaction energy for the oxidized form of 2,6-DBEAQ to form atetrahedral intermediate.

Because the relative stability of the reduced form over the oxidizedform cannot be completely explained by looking at only thethermodynamics of the final products, we investigated the thermodynamiclandscape of possible intermediates. As the oxidized form is moreelectron deficient, we estimate the susceptibility to chain loss byevaluating the energy of a hydroxide-substituted intermediate of theDBEAQ molecule (Scheme 4). This intermediate should more easily form forthe oxidized form. As done previously with bromination ofanthraquinones,⁷ the energy is estimated by evaluating the relativeenergy of the tetrahedral intermediate to that of the oxidized molecule(right-most column of Table 2). Here, we see that 2,6-DBEAQ is the moststable isomer, whereas 1,2- and 1,8-DBEAQ are less stable than2,6-DBEAQ, which is consistent with experimental observation. Based onthese findings, it appears that DBEAQ stability against alkyl chain lossis highly sensitive to the exact positioning of the two alkyl chains,particularly with respect to the ketone groups on the anthraquinonecore.

Example 4. Temporal Cycling of with 2,6-DBEAQ

2,6-DBEAQ has superior chemical stability over other quinones reportedto date, notably including 2,6-DHAQ. As a direct probe of the effect ofO-alkylation of anthraquinone on its chemical stability, 2,6-DBEAQ wascycled potentiostatically in a volumetrically unbalancedcompositionally-symmetric cell configuration. The symmetric cellconfiguration, in which the two sides have the same electrolytecomposition, is a simple and direct probe of chemical andelectrochemical stability of redox flow battery reactants.⁸ Any observedcapacity fade is directly related to deactivation of the reactant on thecapacity-limiting side in either its oxidized or reduced state because(1) there is negligible reactant crossover with symmetric compositions;(2) changes to membrane resistance do not result in temporal capacityvariations in potentiostatic cycling; and (3) the unbalanced volumespermit the capacity-limiting side to be taken to its limiting states ofcharge despite potential side reactions.

Potentiostatic symmetric cell cycling was performed according to areported procedure⁸ in order to assess the temporal chemical stabilityof DBEAQ, independent of any variations in apparent capacity as a resultof changes to membrane resistance, and in a configuration where there isnegligible water or reactant crossover due to the same DBEAQconcentration being used in both electrolytes. 7.5 mL of 2,6-DBEAQ wasfully reduced (i.e., charged to 100% state of charge, or SOC) against aposolyte of excess potassium ferrocyanide and then mixed with 7.5 mL ofoxidized 2,6-DBEAQ (0% SOC) to afford a 2,6-DBEAQ electrolyte at 50%SOC. 5 mL of the resulting electrolyte was used as the capacity-limitingside of a volumetrically unbalanced compositionally-symmetric cell,while the remaining 10 mL was used as the non-capacity-limiting side,i.e., with twice the nominal capacity of the capacity-limiting side. Thecapacity-limiting side was then cycled potentiostatically between 0 and˜100% SOC with potential limits of ±0.2 V versus the non-capacitylimiting side, switched when the absolute value of the current densitydecayed to 2 mA/cm², which is ˜3× higher than the background current.

Full cell cycling (i.e. with a ferro/ferricyanide-based posolyte) wasperformed with the same flow cell hardware but with a FUMASEP® E-620(K)membrane due to its high conductivity and low permeability of DBEAQ andferricyanide compared to other membranes. For studies at pH 14, theposolyte volume was 30 mL, and its composition, when assembled, was 0.20M potassium ferrocyanide, 0.08 M potassium ferricyanide, and 1 M KOH.The negolyte was prepared by dissolving 0.1 M 2,6-DBEAQ in its oxidizedform in 1.2 M KOH solution, resulting in 0.1 M 2,6-DBEAQ, 1.2 M K+, and1 M OH⁻ electrolyte solution. For studies at pH 12, both electrolytescomprised 10 mM OH⁻; the pH was checked both with a pH meter and pHpaper and adjusted where necessary. For all full cell studies, thenegolyte was assembled in the fully discharged state. Galvanostaticcycling was performed at ±0.1 A/cm² at room temperature, with voltagelimits of 0.6 and 1.4 V. To obtain the polarization curves, the cell wasfirst charged to the desired state of charge and then polarized vialinear sweep voltammetry at a rate of 100 mV/s. This method was found toyield polarization curves very close to point-by-point galvanostaticholds, yet to impose minimal perturbation to the SOC of thesmall-electrolyte-volume cell. Electrochemical impedance spectroscopy(EIS) was performed at SOCs between 10 and 100% at open-circuitpotential with a 10 mV perturbation and with frequency ranging from 1 to300,000 Hz.

FIG. 8A shows a schematic of the cell setup and the results ofunbalanced compositionally-symmetric cell cycling for 2,6-DBEAQ at 0.10and 0.65 M for durations of 13 and 26 days, respectively (FIGS. 8B and8C). In both cases, 2,6-DBEAQ exhibited temporal capacity fade rates of<0.01%/day or <3.0%/year, which suggests a loss mechanism that is firstorder in 2,6-DBEAQ concentration. In contrast, 2,6-DHAQ cycled in asimilar cell configuration demonstrated a much higher temporal fade rateof 5%/day (FIG. 9 ). Temporal capacity fade rates for neutralorganic/organometallic RFBs have been summarized elsewhere.⁹ Mostsystems show fade rates in the range of 0.1-3.5%/day. The temporal faderate of 2,6-DBEAQ is the lowest ever reported for a quinone-basedelectrolyte, with the lowest previously reported being for9,10-anthraquinone-2,6-disulfonate (AQDS) at 0.1-0.2%/day.^(10,11) It ison par with that of a recently reported viologen-based flow battery,⁹which exhibited the highest capacity retention rate for any flow batteryin the absence of rebalancing processes, with a temporal capacity faderate of <0.01%/day in symmetric cell testing.⁸

The origin of the higher chemical stability of 2,6-DBEAQ as compared to2,6-DHAQ is not completely understood; however, we highlight here a fewkey distinctions that may be responsible. Notably, it is the reducedform of 2,6-DHAQ that has been shown in a previous study to be involvedin the loss of redox activity, whereas the reduced form of 2,6-DBEAQ isshown here to be quite stable even at temperatures up to 95° C. Ourfirst observation is that, because the redox potential of 2,6-DBEAQ is˜200 mV higher than that of 2,6-DHAQ, 2,6-DBEAQ should be more stablethermodynamically in its reduced form. Secondly, at high pH, we expectboth the reduced hydroquinone core and solubilizing groups to bedeprotonated and therefore negatively charged in both molecules. Thecloser proximity of the negatively charged deprotonated hydroxyl groupsin 2,6-DHAQ should lead to a larger intramolecular Coulombic repulsionforce, which may contribute to this destabilization of the reduced formof the molecule. Finally, the greater susceptibility to the formation ofsemiquinone radicals in 2,6-DHAQ, as discussed above, may also beinvolved in its decreased stability.

The alkyl chain functionalization, while drastically improving lifetime,does not avoid decomposition altogether. By performing elevatedtemperature chemical stability studies, we have identified that thecleavage of γ-hydroxybutyrate is involved in the decomposition of theoxidized form of 2,6-DBEAQ (FIGS. 10A-10C and FIGS. 11A-11D), and wehave characterized the time course of γ-hydroxybutyrate cleavage at pH12 and pH 14 and at 0.1 M and 0.5 M concentration (FIG. 12 and FIGS.13A-13C). The results suggest that the half-life of 2,6-DBEAQ at roomtemperature, pH 14, and 0.1 M concentration in the oxidized form is onthe order of 5 years, with substantially slower decomposition in thereduced form relative to the oxidized form and at pH 12 relative to pH14; these observations are consistent with long redox flow batterylifetime at typical operating cell conditions.

Example 5. Membrane and Full Cell Studies Using 2,6-DBEAQ

Ion-conductive membranes play a critical role in RFB systems ingoverning the transport of counter ions across the membranes (ionicconductivity) and the simultaneous transport of redox-active species(membrane crossover). In a compositionally-asymmetric cell, e.g.2,6-DBEAQ-ferrocyanide system, capacity fade rate can be greatlyexacerbated by the irrecoverable crossover of the reactant to the otherside of the electrolyte.¹² To limit capacity fade by this mechanism,while retaining high permselectivity, we surveyed a range ofcation-exchange membranes, including the industry standard NAFION®-basedperfluorosulfonic acid (PFSA) membranes, by characterizing both their K⁺conductivity and permeability of 2,6-DBEAQ and ferricyanide (morepermeable than ferrocyanide). FUMASEP® E-600 series membranes, whichcomprise a non-fluorinated, sulfonated polyaryletherketone-copolymerbackbone, delivered the best performance. The membrane displayed a lowarea specific resistance (ASR) of ˜1 Ω·cm² in 1 M K⁺ solution, which iscomparable to Nafion 212. It also showed an extremely low 2,6-DBEAQ andferricyanide permeability of 5.26×10⁻¹³ cm²/s and 4.4×10⁻¹² cm²/s,respectively (FIGS. 14A-14B), which are at least an order of magnitudelower than Nafion 212 systems. To further prove the low permeability ofredox-active species, we constructed a low concentration2,6-DBEAQ-ferri/ferrocyanide full cell at pH 14, using a FUMASEP®E-620(K) membrane. Over a period of 4-day (˜880 cycles) galvanostaticcycling testing, the cell showed immeasurably low capacity fade and acurrent efficiency around ˜99.94% (FIG. 15 ). The low 2,6-DBEAQpermeabilities measured ex situ using the nutating table setup translateto negligible crossover of 2,6-DBEAQ in an operating cell, andadditional contributions to crossover that are not present in thenutating table tests, such as electromigration and pressure from activepumping of electrolytes¹³, are unimportant. These low permeabilities,together with the fact that FUMASEP® E-600-series membranes do notcontain fluorine, implies that their use in a DBEAQ-ferri/ferrocyanidefull cell affords a potentially robust configuration for a long-lastingRFB with low power cost (in $/kW) for fully installed systems.

To reduce the corrosivity of the system and the ferricyanidedecomposition rate, which is exacerbated at high pH^(14,15), weperformed the full cell tests of 2,6-DBEAQ at a more moderateelectrolyte condition of pH 12. Symmetric cell cycling of 1,2-(FIG. 16A)and 1,8-DBEAQ (FIG. 16B) at pH 12 showed higher temporal faderates—greater than 0.1 and 10%/day, respectively—than 2,6-DBEAQ at thesame pH (<0.01%/day, FIG. 17 ) and were therefore not tested in fullcell experiments. Theoretical calculations suggest that these isomersare more thermodynamically susceptible to hydroxide or water-inducedγ-hydroxybutyrate cleavage than 2,6-DBEAQ (Table 2). Given the 0.6 Msolubility measured for 2,6-DBEAQ at pH 12 (Table 3), subsequent celltests were performed at 0.5 M in order to examine the performance of afull cell with reasonable energy density.

TABLE 3 Effect of solubility and full cell voltage when paired against apotassium ferrocyanide-based of posolyte on theoretical energy densitiesof 2,6- DHAQ and different DBEAQ isomers, with the solubility ofpotassium ferrocyanide estimated to be 0.5M at pH 14 and 1.25M at pH 12.Solubilities for potassium ferricyanide and 2,6-DBEAQ at pH 14 and 12are reported in Table 1. For 2,6-DBEAQ and 2,6-DHAQ, cell voltages wereobtained from the OCV at 50% SOC from full-cell tests, whereasdifferences in CV-obtained redox potentials between DBEAQ and potassiumferri/ferrocyanide were used for 1,2- and 1,8-DBEAQ. Solubility CellTheoretical Energy (M) at pH Voltage Density (Wh/L) Negolyte 14 (pH 12)(V) at pH 14 (pH 12) 2,6-DHAQ 0.06 1.20 11.4 2,6-DBEAQ 1.1 (0.60) 1.0511.5 (17.2) 1,2-DBEAQ 0.9 1.04 10.9 1,8-DBEAQ 0.75 1.01 10.2

From the Pourbaix diagram of 2,6-DBEAQ (FIGS. 18A-18B), its reductionpotential becomes pH independent above pH 11.5 and is not expected tochange during cell cycling. Polarization and capacity utilizationmeasurements with a negolyte containing 0.5 M 2,6-DBEAQ are shown inFIGS. 19A-19D. Polarization studies (FIG. 19A) at room temperatureshowed a near-linear relationship between current density and voltage atcurrents close to the open-circuit voltage (OCV), which increased from0.97 V at 10% SOC to 1.12 V at 100% SOC (FIG. 19B). Between 80% and 90%of the polarization area-specific resistance (ASR) is accounted for bythe high-frequency resistance measured using EIS, which largely reflectsmembrane resistance. A peak galvanic power density of 0.24 W/cm² wasrealized at 100% SOC (FIG. 20 ). This power density is about half ofthat previously reported in a 2,6-DHAQ-ferrocyanide cell¹⁶, owing to thehigher OCV of the latter (1.20 V as opposed to 1.05 V at 50% SOC) andsmaller area-specific resistance (0.858 Ωcm² as opposed to 1.2 Ωcm² at50% SOC). When voltage and current have a linear, Ohmic relationship,the peak galvanic power density is given by p_(max)=V_(oc) ²/r, whereVOC is the open-circuit potential and r is the area-specific resistance.The low permeability of the FUMASEP® membrane to the fastest-crossingspecies (i.e., 200 years required for 50% loss through crossover offerricyanide) permits, in principle, a four-fold reduction in themembrane thickness, which would raise the power density significantly.

In order to avoid temporal variations in accessible capacity during fullcell cycling caused by changes in membrane resistance⁸, eachgalvanostatic half-cycle was finished with a potential hold at thepotential limit (1.4 V after charge, 0.6 V after discharge), until themagnitude of the current density fell below 2 mA/cm². Over a 5-day testperiod, a capacity fade rate of 0.05%/day or 0.001%/cycle was observed(FIG. 21A). A parallel cycling test was performed in which apotentiostatic charge-discharge cycle was executed after every 20galvanostatic cycles; a capacity fade rate of 0.04%/day was observed inthat case (FIG. 22B). It has been shown that capacity retention ratesusing both cycling protocols yield virtually identical results.⁸

These full cell measurements, however, showed roughly 4-fold increase incapacity fade compared with the <0.01%/day observed during symmetriccycling tests (FIGS. 8B-8C), suggesting additional capacity fademechanisms not observed during symmetric-cell studies. In-depth chemicaland electrochemical analysis (FIGS. 22A-22B) was performed to probe thechemical decomposition and crossover of DBEAQ from the capacity limitingside. Electrolytes in a cell compositionally identical to that in FIG.21A but cycled for 11 days were subjected to NMR (FIG. 22A) and CV (FIG.22B) analysis. From these results, no evidence of DBEAQ decompositionand crossover was found after examining the negolyte and posolyte beforeand after cycling. Based on the detection limit of NMR (0.1 mM DBEAQ)and CV techniques under the experimental conditions chosen, the upperlimit of the capacity fade rate caused by 2,6-DBEAQ decomposition and/ormembrane crossover was ˜0.01%/day, similar to our symmetric cell study.We therefore hypothesize that other capacity fade mechanisms, such asprecipitation of 2,6-DBEAQ in the posolyte after crossing over, might beoperative but untraceable by NMR and CV techniques due to the slowcapacity fade rate, which corresponds to a total loss of <0.5% of2,6-DBEAQ over a 6-day testing period. One other such mechanism might beleakage of the negolyte due to poorer adhesion between the thinnerFUMASEP® membrane and the Viton gasket in the full cell than betweenNAFION® N117 and the gasket in the symmetric cell. Indeed, the totalcapacity fade in FIG. 21B corresponds to a total loss over the entire6-day cycling period of ˜10 μL of negolyte volume, which is roughly onefifth of a droplet. When translated to an equivalent current density(0.5 μA/cm²), this negolyte loss rate is well within expectation forseepage of the electrolyte into spaces between the gaskets and/orinterface between the membrane and gaskets, as compared to an analogousleak rate estimated from a previous study of capacity fade in ananthraquinone-based flow battery with this cell architecture (0.09-0.12mA/cm²).¹⁴

Example 6. Permeability of Oxidized 2,6-DBEAQ Across Membranes

The permeability of the oxidized form of 2,6-DBEAQ and potassiumferricyanide across a FUMASEP® E-620(K) membrane was evaluated with alab-made two-compartment cell. In the first case, the donating side wasfilled with a solution of 2,6-DBEAQ (0.1 M) in 1.6 M KOH, while thereceiving side was filled with 1.6 M KOH. For potassium ferricyanidepermeability, the donating side was filled with potassium ferricyanide(0.3 M) in 1 M KOH, while the receiving side was filled with 1.9 M KOH.Both compartments had the same volume. The cell was continuouslyagitated on a nutating table. At different time intervals, aliquots weretaken from the receiving side, diluted, characterized by UV-Visspectrophotometry, and replaced by fresh KOH solution. The electrolytevolumes on both sides were checked periodically to ensure that there wasnegligible water flux across the membrane, which might affect theapparent reactant permeability. The concentration was calculated from acalibration curve and the permeability of 2,6-DBEAQ was calculated basedon Fick's law using the following equation:

$P = \frac{\Delta{\ln\left( {1 - \frac{2c_{t}}{c_{0}}} \right)}\left( \frac{V_{0}l}{2A} \right)}{\Delta t}$where P is permeability (cm²/s), A is the effective membrane area (cm²),t is elapsed time (s), c_(t)(mol/L) is the concentration of activespecies in the receiving side at time t, V_(o) is the volume of thesolution in either compartment (5 cm³), l is the thickness of themembrane (˜20 μm), c_(o) is the concentration of 2,6-DBEAQ in thedonating side at time zero (0.1 mol/L), and Δ represents a finitedifference.

Example 7. Aqueous Flow Battery with 2,6-DBEAQ the Negative Electrolyteand Potassium Ferrocyanide as the Positive Electrolyte

A flow battery was constructed using a solution of 2,6-DBEAQ produced asdescribed in Example 1. Cell hardware from Fuel Cell Tech. (Albuquerque,N. Mex.) was used to assemble a zero-gap flow cell configuration,similar to a previous report (supplemental information in the paper byK. Lin et al. Science 349, 1529 (2015)). Pyrosealed POCO graphite flowplates with serpentine flow patterns were used for both electrodes. Eachelectrode had a 5 cm² geometric surface area covered by a stack of threesheets of Sigracet SGL 39AA porous carbon paper pre-baked in air for 8 hat 400° C. A sheet of pretreated NAFION® 212 membrane served as theion-selective membrane between the carbon electrodes. Pretreatment ofthe NAFION® 212 membrane was performed by first heating it at 80° C. inde-ionized water for 20 minutes and then soaking in 5% hydrogen peroxidesolution at room temperature for 35 minutes. These pre-treated membraneswere stored in 0.1 M KOH solution at room temperature. The outer portionof the space between the electrodes was gasketed by Teflon sheets withthe area over the electrodes cut out. Torque applied during cellassembly was 90 lb-in on each of 8 bolts. The electrolytes were fed intothe cell through PFA tubing, at a rate <60 ml/min controlled byCole-Parmer Micropump gear pumps. The cell was run inside anitrogen-filled glove box with an O₂ partial pressure of about 1 to 2ppm. Cell polarization measurements, impedance spectroscopy, andcharge-discharge cycling were performed using a Biologic VSP 300potentiostat. The posolyte volume and composition when assembled was 30mL of 0.22 M potassium ferrocyanide, 0.02 M potassium ferricyanide and 1M KOH. The capacity-limiting negolyte was prepared by dissolving 1.2mmoles of 2,6-DBEAQ (0.5 g) in its oxidized form in 1.2 M KOH solution(6 mL) resulting a 0.2 molar 2,6-DBEAQ solution.

For a full cell study, the electrolytes were assembled in the fullydischarged state, deaerated and brought into a nitrogen-filled glovebox.The oxygen level was maintained at <2 ppm during cycling. Galvanostaticcycling was performed at ±0.1 A/cm² at room temperature, with voltagelimits of 0.6 and 1.4 V, controlled by a Gamry 30K Booster potentiostat.

The open-circuit voltage was 1.02 V at 50% state of charge with respectto the negolyte. Our experiments have shown that the loss of capacity ofa cell based on a 2,6-DBEAQ negative electrolyte was lower than that ofa comparable cell based on a 2,6-dihydroxyanthraquinone negativeelectrolyte.

Example 8. Redox Chemistry of 2,6-DBEAQ Vs. 2,6-DHAQ

Following our previous study on 2,6-DHAQ¹⁷ and literature reports^(1,18)on quinone electrochemistry in the absence of proton donors, wesimulated the CV of 2,6-DBEAQ as representing two reversible electrontransfer steps at two different potentials E₁ and E₂. The separationbetween simulated redox potentials for the first (E₁) and second (E₂)electron reductions for 2,6-DBEAQ is considerably narrower than thecorresponding peak separation for 2,6-DHAQ¹⁷ (6 mV vs. 60 mV). Animportant implication of this difference is that there is a smallerdriving force for the formation of semiquinone radicals between fullyoxidized and fully reduced 2,6-DBEAQ species than there is for 2,6-DHAQ.In order to demonstrate why this is the case, we first explain theeffect of E₁-E₂ separation on the reduction mechanism and then draw aconnection between the reduction potential difference and thesemiquinone concentration.

The pH dependence of the proton-coupled electron transfer (POET) redoxreaction for a generalized anthraquinone (AQ) is often visualized by thenine-membered square scheme (FIG. 7 ). The horizontal directionindicates electron transfer (ET), and the vertical direction indicatesproton transfer (PT or protonation). Under strongly alkaline conditions,where the pH is higher than the second pK_(a) (pK_(a2)) of the reducedAQ (AQ²⁻), no protonation of the semiquinone anion will take place, andreduction proceeds via two ET steps, from AQ to a semiquinone (AQ-^(●))at E₁, and from AQ-^(●) to AQ²⁻ at E₂ which is typically <E₁. Dependingon the value of E₁-E₂, the mechanism of AQ reduction in alkalineconditions can either be a concerted one-step, two-electron process ortwo single-electron processes. When E₂ becomes more negative relative toE₁, a stepwise process via a semiquinone radical is followed, as in AQreduction in aprotic solvents.^(1,19) Under these conditions, acomproportionation reaction, where AQ and AQ²⁻ combine to generateAQ-^(●) semiquinone (Equation 3) is thermodynamically favorable. Theequilibrium constant for this comproportionation (K_(eq_comp)) can beobtained by dividing the equilibrium constants of E₁ (Equation 4) bythat of E₂ (Equation 5), which now becomes Equation 6.¹⁸ The ratiobetween semiquinone concentration and the geometric mean of theconcentrations of the oxidized and reduced forms of AQ can now be seenas an increasing function of E₁-E₂.

$\begin{matrix}\left. {{AQ} + e^{-}}\rightleftharpoons{AQ}^{- \cdot} \right. & \left( {E{q.\ 1}} \right) \\\left. {{AQ^{- \cdot}} + e^{-}}\rightleftharpoons{AQ}^{2 -} \right. & \left( {{Eq}.\ 2} \right) \\\left. {{AQ} + {AQ^{2 -}}}\rightleftharpoons{2AQ^{- \cdot}} \right. & \left( {{Eq}.\ 3} \right) \\{{E_{1} = {\frac{RT}{ZF}\ln\; K_{eq1}}};{K_{eq1} = {\frac{\left\lbrack {AQ^{- \cdot}} \right\rbrack}{\left\lbrack {AQ} \right\rbrack} = {\exp\;\left( {\frac{zF}{RT}E_{1}} \right)}}}} & \left( {{Eq}.\ 4} \right) \\{{E_{2} = {\frac{RT}{ZF}\ln\; K_{eq2}}};{K_{eq2} = {\frac{\left\lbrack {AQ^{2 -}} \right\rbrack}{\left\lbrack {AQ^{- \cdot}} \right\rbrack} = {\exp\;\left( {\frac{zF}{RT}E_{2}} \right)}}}} & \left( {{Eq}.\ 5} \right) \\{K_{eq\_ comp} = {\frac{K_{eq1}}{K_{eq2}} = {\frac{\left\lbrack {AQ^{- \cdot}} \right\rbrack^{2}}{\lbrack{AQ}\rbrack \cdot \left\lbrack {AQ}^{2 -} \right\rbrack} = {\exp\;\left\lbrack {\frac{zF}{RT}\left( {E_{1} - E_{2}} \right)} \right\rbrack}}}} & \left( {{Eq}.\ 6} \right)\end{matrix}$

This equation directly relates the concentration of semiquinone speciesto the E₁-E₂ separation and suggests that K_(eq_comp) for semiquinoneformation is 2.83 times higher for 2,6-DHAQ than it is for 2,6-DBEAQ forthe same AQ:AQ²⁻ ratio, based on their respective E₁-E₂ separations.

The differences in energy between the first and second reduction canalso be estimated with electronic structure calculations. Conformers ofthe oxidized, one-electron reduced, and two-electron reduced forms of2,6-DHAQ and 2,6-DBEAQ were generated with the UFF force field usingRDKit.⁶ The redox active sites of the one-electron and two-electronreduced forms were paired with one and two potassium cations,respectively. The deprotonated carboxyl and hydroxyl groups were alsopaired with potassium cations. This pairing is done to approximate theion pairing behavior of the molecules in solution in lieu of an explicitsolvent, as has been done previously.²⁰ The relative energies of thefirst and second reduction were calculated with both semiempiricalmethods (PM7 COSMO) and density functional theory (DFT) methods in apolarizable continuum model (PCM) implicit solvent (B3LYP/6-311+G(d,p)).The DFT single point energies were evaluated at the B3LYP/6-31G(d) (noPCM) minima. Typically, when calculating reduction potentials, acalibration scheme is employed,¹⁹ or the relative energetics of allmolecules are computed relative to a fixed molecule where theexperimental result is known¹⁹; the latter strategy is used in this casewith 2-6 DHAQ used as the internal standard. We find that using DFT isnecessary to qualitatively predict these differences (though the shiftis overestimated) in the first and second reduction. This suggests thatsuch a behavior can be screened for in future studies, at least forquantitative estimation of semiquinone presence.

TABLE 4 Estimated difference in potential between the first and thesecond reduction for 2,6-DHAQ and 2,6-DBEAQ. B3LYP/6-311+G(d,p) PCMNegolyte Estimated E₁-E₂ (V) 2,6-DHAQ* 0.060 2,6-DBEAQ −0.133 *Note thatthe energy differences are calibrated so that the differences for2,6-DHAQ are fixed to experiment.

Example 9. Decomposition of 2,6-DBEAQ

Chemical stability studies at elevated temperature indicate that theoxidized form of 2,6-DBEAQ is susceptible to decomposition byγ-hydroxybutyrate cleavage (Scheme 5), as confirmed by ¹H NMR.²¹Notably, cleavage of a second γ-hydroxybutyrate moiety leads to2,6-dihydroxyanthraquione (2,6-DHAQ), whose capacity fade rate, has beenreported to be 5%/day.^(21,22) The rate of 2,6-DBEAQ decomposition hasbeen shown to be considerably slower at pH 12 relative to pH 14 at 95°C. and 0.1 M concentration (FIGS. 23E, 23F, and 24A-24D).²¹ Noting theinfluence of pH on the degradation rate and that both hydroxide andcarboxylate may act as nucleophiles in the decomposition (Scheme 5), wetherefore expect further enhancement in stability with lower pHoperating conditions and with suppression of the carboxylate-mediatedintramolecular nucleophilic substitution reaction. We designed ananalogue of 2,6-DBEAQ but with phosphonate-terminated functional groups,DPPEAQ, appearing to deliver both of these desired outcomes.

Example 10. Synthesis of DPPEAQ

2,6-dihydroxyanthraquinone (2,6-DHAQ) was purchased from AK Scientific.All other chemicals were purchased from Sigma Aldrich. All chemicalswere used as received unless specified otherwise.

2,6-DHAQ (10 mmol) was mixed with anhydrous K₂CO₃ (40 mmol) and diethyl(3-bromopropyl)phosphonate (30 mmol) in DMF (50 mL). The reactionmixture was then heated to 100° C. overnight. After removing thesolvent, the solid was washed thoroughly with DI water. The product wasanalyzed by ¹H NMR and used for the next reaction step without furtherpurification. The ester precursor of DPPEAQ was dissolved indichloromethane (100 mL), and trimethylsilyl bromide (TMSBr) (100.0mmol) was added. After 15 h stirring at room temperature, the solventand excess TMSBr were distilled off. The mixture was washed thoroughlywith DI water and hexane, then vacuum dried to yield yellow solid. Finalyield: 98%.

DPPEAQ was synthesized by the route illustrated in Scheme 6, whichpresents several advantages that make it particularly suitable for gridstorage. The simple synthesis uses only two steps, an O-alkylationreaction and hydrolysis of the ester, to introduce the highly solublephosphonic acid terminal groups. Both synthesis steps achieved >98%yield, facilitating scale-up without the need for moreresource-intensive purification such as chromatography and thuspresenting a feasible pathway for large scale industrial manufacturing.

The ¹H NMR spectrum of(((9,10-dioxo-9,10-dihydroanthracene-2,6-diyl)bis(oxy))bis(propane-3,1-diyl))bis(phosphonicacid) (DPPEAQ) is shown in FIG. 25 .

Example 11. Permeability and Solubility Tests of DPPEAQ

Permeability Measurements

The permeability of the electroactive molecules was measured in alab-made two compartment diffusion cell. A 0.1 M solution of DPPEAQ inKOH at pH 9 and pH 14 was placed on the donating side and paired with0.2 M KCl at pH 9 and pH 14 solution on the receiving side, to balanceboth the potassium concentration and the ionic strength between sides.To keep the solutions under agitation, the cell was placed on a nutatingtable. The increase of DPPEAQ concentration in the receiving side wasmeasured as a function of time with UV-visible absorptionspectrophotometry (Ocean Optics Flame-S Spectrometer Assembly). For eachtime point, a 400 μL aliquot of the solution on the receiving was taken,diluted, measured by UV-Visible and replaced by fresh 0.2 M KClsolution.

Due to a very low crossover and to the detection limit of thespectrometer we were only able to estimate a maximum value of DPPEAQpermeability. According to the derivation of Fick's Law as reported inour previous paper²¹, DPPEAQ permeability must be lower than 3.4×10⁻¹³cm²/s. Ferricyanide permeability has been measured and publishedpreviously for this membrane and it is equal to 4.4×10⁻¹² cm²/s.²¹

Solubility Tests

The solubility limit of DPPEAQ was measured in the oxidized form byadding the potassium salt of DPPEAQ (prepared by reacting DPPEAQ withpotassium hydroxide in water) until no further solid could be dissolved.The mixture was adjusted to pH 9. After filtering the mixture through aPTFE 0.45 μm syringe filter, a saturated solution of DPPEAQ at pH 9 wasobtained. The saturated solution was then diluted by a known amountwhile maintaining a pH of 9, and the concentration was evaluated byUV-Vis (Agilent Cary 60 spectrophotometer). The concentration wascalculated according to a pre-calibrated absorbance-concentration curveof known concentrations of DPPEAQ at pH 9. The resulting value of thesolubility of the oxidized form of DPPEAQ at pH 9 is 0.75 M.

The solubility of the oxygen-sensitive reduced form of DPPEAQ was notmeasured but was assumed to be higher than the oxidized form because:(1) no precipitation after full electrochemical reduction of DPPEAQ wasobserved, and (2) the increased negative charges of DPPEAQ in thereduced form compared to DPPEAQ in the oxidized form is expected torender quinone-quinone interactions even more unfavorable and increaseits solubility.

Whereas the solubility of 2,6-DBEAQ at pH 9 (<35 mM) is impracticallylow for use in a cell, the phosphonate functional group affords DPPEAQ amuch higher solubility at pH 9 (0.75 M) to enable the operation ofDPPEAQ under milder conditions. When similar elevated temperaturechemical studies were performed for DPPEAQ at pH 9 as well as at pH 12,no significant decomposition was observed in either case (FIGS.23A-23D). 2,6-DBEAQ and DPPEAQ do, however, exhibit a similar extent ofdecomposition at pH 14 (FIGS. 24A-24D). The differing relativestabilities of DPPEAQ and 2,6-DBEAQ at pH 12 (at 95° C. and 0.1 Mconcentration), in contrast to the similar extent of their observeddecomposition at pH 14, may be due to a competition between multipledecomposition mechanisms that vary in a pH-dependent manner. Theconcentration of hydroxide ions at pH 14 is tenfold higher than theconcentration of either molecule at 0.1 M, as opposed to tenfold lowerat pH 12. In particular, we propose a hydroxide-mediated nucleophilicsubstitution reaction (S_(N)Ar or S_(N)2) that dominates thedecomposition of both molecules at pH 14. However, at a lower pH of 12,an intramolecular reaction with carboxylate acting as the nucleophiledominates in the decomposition of 2,6-DBEAQ, while no analogousintramolecular reaction of DPPEAQ at pH 12 can take place due to muchweaker nucleophilicity of bulked phosphonate compared with carboxylate.The difference in the dominant mechanisms of 2,6-DBEAQ decomposition atpH 12 and 14 that we are proposing is also consistent with theobservation that the initial rate of degradation is not 100-fold slowerat pH 12 relative to pH 14. Unlike the oxidized form of DPPEAQ, itsreduced form, obtained by electrochemical reduction, exhibits robustchemical stability even at pH 14 (FIGS. 26A-26C). Collectively, theseresults suggest the high stability of DPPEAQ between pH 9 and 12. Inaddition, the operability at lower pH serves to decrease the corrosionof battery systems and enable the use of less expensive materials.

Example 12. Cyclic Voltammetry (CV) and Rotating Disk Electrode (RDE)Measurements of DPPEAQ

Glassy carbon was used as the working electrode for all three-electrodeCV tests. RDE experiments were conducted using a Pine InstrumentsModulated Speed Rotator AFMSRCE equipped with a 5 mm diameter glassycarbon working electrode, a Ag/AgCl reference electrode (BASi,pre-soaked in a 3 M KCl solution), and a graphite counter electrode. Thediffusion coefficient of the oxidized form of DPPEAQ was calculatedusing the Levich equation, which relates the mass-transport-limitedcurrent to the number of electrons transferred (n), the area of theelectrode (A), and the concentration of redox-active species in theelectrolyte (C), by plotting the mass-transport-limited current againstthe square root of the rotation rate (FIGS. 27A-27B) with the followingparameters: n=2, F=96,485 Coulombs/mol, A=0.196 cm², C=5 mM, kinematicviscosity of 1 M KOH v=1.08×10⁻⁶ m²/s. The resulting value of thediffusion coefficient for the oxidized form of DPPEAQ is 1.37×10⁻⁶cm²/s.

Based on cyclic voltammetry (CV), DPPEAQ exhibits a reversible redoxpeak at −0.47 V vs. SHE (E_(1/2)) in pH 9 unbuffered aqueous solutionand at −0.49 V vs. SHE (E_(1/2)) in pH 12 unbuffered aqueous solution(FIG. 28A). In contrast, in buffered pH 9 and pH 12 solutions, DPPEAQshows redox potentials of −0.39 vs. SHE (E_(1/2)) and −0.47 V vs. SHE(E_(1/2)) respectively (FIG. 29 ), conforming to the pH-dependent CVbehavior of quinones.¹ During the cell cycling process, theproton-coupled redox reactions of quinones in water can alter the pH, asthe reduction of quinones in aqueous solution consumes protons, therebyincreasing pH, whereas oxidation of hydroquinones in aqueous solutionreleases protons, thereby decreasing pH. These pH fluctuations should bereversible during cell cycling.

Example 13. Cycling Stability Measurements of DPPEAQ

To prove the pH reversibility and cycling stability, a cell wasassembled with a negolyte including 0.1 M DPPEAQ in 1 M KCl, the pH ofwhich was monitored by a pH probe immersed in the solution, and aposolyte comprising 0.1 M K₄Fe(CN)₆ and 0.01 M K₄Fe(CN)₆ in 1 M KCl,separated by a FUMASEP® E-620(K) membrane (FIGS. 28B-28C). The pH ofboth sides was adjusted to 9 by adding a trace of KOH.

Operating at pH 9 enhances the stability of the posolyte as well as thenegolyte, as the decomposition rate of ferricyanide, which is typicallyused as a posolyte in neutral and alkaline batteries, is increased athigh pH.^(15,23) The cell was cycled at a constant current density of±20 mA/cm² for 16 cycles using voltage cut-offs of 0.4 V and 1.2 V.During the first charge cycle, the negolyte pH increased from 9.2 to12.2. After the first discharge cycle, the pH decreased to 9.9 insteadof 9.2. We attribute the increase in pH in the discharged state to theconsumption of atmospheric oxygen dissolved in the solution andreservoir. When hydroquinones are oxidized electrochemically during celldischarge, the increase in pH that occurred during charging isreversible. If, however, hydroquinones, in any protonation state, areinstead oxidized by molecular oxygen, then an irreversible increase inpH occurs. In this case, the quinone serves as a mediator for the oxygenreduction reaction (ORR), which has the effect of increasing the pH.After the first cycle, the current efficiency exceeded 99%, and both thepotential and pH cycles exhibited high reproducibility. The negolyte pHfluctuated reversibly between approximately 10 and 12 during the chargeand discharge process. The discharge capacity did not fade after 16cycles (9.5 hours), indicating that the battery was capable of operatingwith minimal capacity fade, consistent with the demonstrated highchemical stability. The negolyte pH in the charged state remained atapproximately pH 12.2, suggesting that the redox reaction becomes pHindependent at pH values higher than 12.2. In contrast, the negolyte pHin the discharged state continued to slowly increase to 10.4 after 16cycles (9.5 hours), which we attribute to oxygen penetration. Weanticipate that with oxygen penetration, the negolyte pH in thedischarged state will continue to gradually increase and may eventuallyexceed 12.2 in both the charged and discharged states after long-termcycling. The posolyte pH remained stable, as the redox reactions offerri-ferrocyanide do not involve protons. To exclude oxygen penetrationcausing pH increasing, another cell was cycled in the glove box insteadof glove bag. As shown in the FIG. 28D, both the potential and pH cycleswere quite reproducible. The pH in the discharged state stayed around pH9 as the beginning and the pH in the charged state stayed around pH 12,suggesting the feasibility of cell cycling between pH 9 and 12 whenoxygen was eliminated.

The cycling stability of the DPPEAQ electrolyte was also studied by apreviously published symmetric cell method.²⁴ Although the oxidized formof DPPEAQ is readily soluble at pH 9, proton-coupled electron transferraises the pH of the electrolyte when it is reduced. Such pHfluctuations can produce misleading results in symmetric cell testingunless the starting pH is above the cutoff point for proton-coupledelectron transfer. The experiment shown in FIG. 30A demonstrates thecapacity fade behavior of a symmetric cell at pH 13, using +−200 mV toaccess >99% of the theoretical capacity of the 4.5 mL of 0.1 M limitingelectrolyte, the cell was cycled for 6 days at pH 13, averaging0.02%/day of capacity fade (FIG. 28A).

Example 14. Full Cell Measurements of DPPEAQ in an Aqueous Flow Battery

High concentration and long-term full cell testing was performed at 20°C. with solutions of 0.5 M DPPEAQ in the negolyte and 0.4 M K₄Fe(CN)₆,0.1 M K₃Fe(CN)₆ in the posolyte, both dissolved in aqueous solution withpH adjusted to approximately 9 by the addition of traces of KOH. Thehigh solubility and high charge of DPPEAQ enable the use of lesssupporting electrolyte without compromising the ionic conductivity ofthe solution. These solutions were pumped through a flow cellconstructed from graphite flow plates and carbon paper electrodes,separated by a FUMASEP® E-620 (K) membrane, which has a highconductivity and low permeability to DPPEAQ and ferricyanide as reportedin the supporting information. The RFB was charged stepwise at constantvoltage (1.5 V) with a 10% increment in the state of charge (SOC) atroom temperature; polarization curves were measured at 10%, 50%, and 90%states of quinone charge (SOC). The OCV at 50% SOC is approximately 1.02V. As the SOC increased from 10% to 90%, the open-circuit potentialincreased from 0.95 V to 1.05 V (FIG. 30B). In the galvanic direction,peak power densities were 0.073 W cm^(−).)and 0.160 W cm^(−nd) at thesesame SOCs, respectively (FIG. 30C). The area-specific resistance (ASR)of the membrane (˜1.3 Ωcm², determined by high-frequency electrochemicalimpedance spectroscopy (EIS) in the full cell) was responsible forapproximately 80% of the ASR of the entire cell (˜1.6 Ωcm², DCpolarization). It is anticipated that a membrane with lower resistivitywill raise the power density significantly.

The cell was cycled at a constant current density of ±0.1 A cm⁻², andeach galvanostatic half-cycle was finished with a potential hold at thepotential limit (1.5 V after charge, 0.5 V after discharge) until themagnitude of the current density fell below 2 mA/cm² to avoid temporalvariations in accessible capacity during full cell cycling caused bychanges in membrane resistance. The cell was cycled for 480 cycles,which required 12.3 days to complete at 100 mA cm⁻². The averagecapacity retention over the 480 cycles was 99.99964%/cycle at an averageCoulombic efficiency greater than 99%, which reflects a capacity faderate of 0.00036%/cycle or 0.014%/day (FIG. 30D). This temporal fade rateis lower than any other RFB chemistry that has been published to date,including a viologen-based flow battery with a capacity fade rate of0.0011%/cycle or 0.033%/day in full cell testing.²⁵ After 12.3 days ofcycling, the pH of the discharged negolyte was measured to haveincreased to nearly 13 as we predicted. Although the cell was run in aglove bag with nitrogen flowing, some oxygen was inadvertentlyintroduced during the initial set-up. As mentioned previously, thisinflux of oxygen would have caused homogeneous oxidation of the chargednegolyte and an associated permanent increase in the pH of the negolyte.The same experiment was also done in the glove box, showing thereversible pH fluctuations during quinone cell cycling when oxygen waseliminated (FIG. 31 ).

In conclusion, by functionalizing an anthraquinone with phosphonategroups, we demonstrate a nearly neutral RFB negolyte with highsolubility and high stability. A full cell at 0.5 M concentration,exhibited an OCV of 1V, high performance and a low capacity fade rate(0.00036%/cycle, 0.014%/day). This is the highest reported capacityretention rate of any aqueous organic RFB chemistry, approaching therequirements for grid storage applications. We also study anddemonstrate the reversible pH fluctuations during quinone cell cycling.This work demonstrates the knowledge and experience gained from studyingthe stability of organic molecules in both oxidized and reduced redoxstates can be used to develop new stable molecule structures to achieveaqueous organic flow battery with long lifetime.

REFERENCES

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Other embodiments are described in the claims.

What is claimed is:
 1. A redox flow battery comprising: a first aqueouselectrolyte comprising a first redox active material; and a secondaqueous electrolyte comprising a second redox active material that is acompound of formula I:

or an ion, salt, or hydroquinone thereof, wherein each of R₁-R₈ isindependently H; halo; optionally substituted C₁₋₆ alkyl; optionallysubstituted C₃₋₁₀ carbocyclyl; optionally substituted C₁₋₉ heterocyclylhaving one to four heteroatoms independently selected from O, N, and S;optionally substituted C₆₋₂₀ aryl; optionally substituted C₁₋₉heteroaryl having one to four heteroatoms independently selected from O,N, and S; oxo; —NO₂; —OR_(a); —N(R_(a))₂; —C(═O)R_(a); —C(═O)OR_(a);—S(═O)₂R_(a); —S(═O)₂OR_(a); —OS(═O)₂OR_(a); —P(═O)R_(a2);—P(═O)(OR_(a))₂; and —OP(═O)(OR_(a))₂; —X₁-L₁-C(O)O—Y₁; —X₂-L₂-C(O)O—Y₂;—X₃-L₃-P(═O)(OY₃)₂; or —X₄-L₄-P(═O)(OY₄)₂; wherein each R_(a) isindependently H; optionally substituted C₁₋₆ alkyl; optionallysubstituted C₃₋₁₀ carbocyclyl; optionally substituted C₁₋₉ heterocyclylhaving one to four heteroatoms independently selected from O, N, and S;optionally substituted C₆₋₂₀ aryl; or optionally substituted C₁₋₉heteroaryl having one to four heteroatoms independently selected from O,N, and S; X₁, X₂, X₃, and X₄ are independently O, S, or CH₂; L₁, L₂, L₃,and L₄ are independently C₁-C₆ alkylene; and Y₁, Y₂, Y₃, and Y₄ areindependently H or optionally substituted C₁-C₆ alkyl, provided that oneand only one of R₁-R₈ is —X₁-L₁-C(O)O—Y₁ or —X₃-L₃-P(═O)(OY₃)₂ and oneand only one of R₁-R₈ is —X₂-L₂-C(O)O—Y₂ or —X₄-L₄-P(═O)(OY₄)₂.
 2. Thebattery of claim 1, wherein X₁ and X₂ are O, and/or wherein each ofR₁-R₈ is independently H; halo; optionally substituted C₁₋₆ alkyl;—X₁-L₁-C(O)O—Y₁;or —X₂-L₂-C(O)O—Y₂, provided R₂ is —X₁-L₁-C(O)O—Y₁ or—X₂-L₂-C(O)O—Y₂, and/or wherein R₂ is —X₁-L₁-C(O)O—Y₁ and R₆ is—X₂-L₂-C(O)O—Y₂, and/or wherein Y₁ and Y₂ are H.
 3. The battery of claim1, wherein X₃ and X₄ are O, and/or wherein each of R₁-R₈ isindependently H; halo; optionally substituted C₁₋₆ alkyl;—X₃-L₃-P(═O)(OY₃)₂; or —X₄-L₄-P(═O)(OY₄)₂, provided R₂ is—X₃-L₃-(═O)(OY₃)₂, or —X₄-L₄-P(═O)(OY₄)₂, and/or wherein R₂ is—X₃-L₃-P(═O)(OY₃)₂ and R₆ is —X₄-L₄-P(═O)(OY₄)₂, and/or wherein Y₃ andY₄ are H.
 4. The battery of claim 1, wherein the compound of formula Iis

or an ion, salt, or hydroquinone thereof; wherein R₂ is —O(CH₂)₃C(O)OH.5. The battery of claim 1, wherein the compound of formula I is

or an ion, salt, or hydroquinone thereof.
 6. The battery of claim 1,wherein the compound of formula I is

or an ion, salt, or hydroquinone thereof.
 7. The battery of claim 1,wherein the compound of formula I is

or an ion, salt, or hydroquinone thereof; wherein R₂ is—O(CH₂)₃P(O)(OH)₂.
 8. The battery of claim 1, wherein the compound offormula I is

or an ion, salt, or hydroquinone thereof.
 9. The battery of claim 1,wherein the second redox active material is the hydroquinone of formulaI, which is oxidized to the corresponding quinone during discharge,and/or wherein the pH of the second aqueous electrolyte is ≥7, and/orwherein the first redox active material comprises bromine, chlorine,iodine, oxygen, vanadium, chromium, cobalt, iron, aluminum, manganese,cobalt, nickel, copper, or lead.
 10. The battery of claim 9, wherein thepH is from 8 to
 13. 11. A compound of formula I:

or an ion, salt, or hydroquinone thereof, wherein each of R₁-R₈ isindependently H; halo; optionally substituted C₁₋₆ alkyl; optionallysubstituted C₃₋₁₀ carbocyclyl; optionally substituted C₁₋₉ heterocyclylhaving one to four heteroatoms independently selected from O, N, and S;optionally substituted C₆₋₂₀ aryl; optionally substituted C₁₋₉heteroaryl having one to four heteroatoms independently selected from O,N, and S; oxo; —NO₂; —OR_(a); —N(R_(a))₂; —C(═O)R_(a); —C(═O)OR_(a);—S(═O)₂R_(a); —S(═O)₂OR_(a); —OS(═O)₂OR_(a); —P(═O)R_(a2);—P(═O)(OR_(a))₂; and —OP(═O)(OR_(a))₂; —X₁-L₁-C(O)O—Y₁; —X₂-L₂-C(O)O—Y₂;—X₃-L₃-P(═O)(OY₃)₂; or —X₄-L₄-P(═O)(OY₄)₂; provided R₂ is—X₁-L₁-C(O)O—Y₁, —X₂-L₂-C(O)O—Y₂, —X₃-L₃-P(═O)(OY₃)₂, or—X₄-L₄-P(═O)(OY₄)₂; wherein each R_(a) is independently H; optionallysubstituted C₁₋₆ alkyl; optionally substituted C₃₋₁₀ carbocyclyl;optionally substituted C₁₋₉ heterocyclyl having one to four heteroatomsindependently selected from O, N, and S; optionally substituted C₆₋₂₀aryl; or optionally substituted C₁₋₉ heteroaryl having one to fourheteroatoms independently selected from O, N, and S; X₁, X₂, X₃, and X₄are independently O, S, or CH₂; L₁, L₂, L₃, and L₄ are independentlyC₁-C₆ alkylene; and Y₁, Y₂, Y₃, and Y₄ are independently H or optionallysubstituted C₁-C₆ alkyl, provided that one and only one of R₁- R₈ is—X₁-L₁-C(O)O—Y₁ or —X₃-L₃-P(═O)(OY₃)₂ and one and only one of R₁-R₈ is—X₂-L₂-C(O)O—Y₂ or —X₄-L₄-P(═O)(OY₄)₂, and provided that: ii. when R₄-R₈are H and R₂, R₃, R₆, and R₇ are —O(CH₂)₃(CH₃), R₁ and R₅ are not both—O(CH₂)₃C(O)OH; iii. when R₄ and R₈ are H and R₂, R₃, R₆, and R₇ are—O(CH₂)₄(CH₃), R₁ and R₅ are not both —O(CH₂)₄C(O)OH; iv. when R₁ and R₈are OH and R₃-R₆ are H, R₂ and R₇ are not both —(CH₂)₃C(O)OH or—(CH₂)₃C(O)OCH₃; v. when R₁, R₄, R₅, and R₈ are NH₂ and R₃ and R₆ are H,R₂ and R₇ are not both —O(CH₂)₃C(O)O(CH₂)₄CH₃; vi. when R₁, R₃-R₅, R₇,and R₈ are H, R₂ and R₆ are not both —OCH(CH₃)C(O)OCH₃, —OCH(CH₃)C(O)OH,or —OCH₂C(O)OCH₃; vii. when R₁, R₃, R₅, and R₇ are H, and R₄ and R₈ are—N(R_(a))₂, R₂ and R₆ are not both —S(CH₂)_(n)C(O)OCH₂CH₃, wherein n is1 to 5; viii. when R₁ and R₄ are OCH₃, and R₅-R₈ are H, R₂ and R₃ arenot both —CH₂CH₂C(O)OH, —CH₂CH₂C(O)OCH₃, or —CH₂CH(C(O)CH₃)C(O)OCH₂CH₃;ix. when R₁ and R₄ are OH, and R₅-R₈ are H, R₂ and R₃ are not both—CH₂CH₂C(O)OCH₃; x. when R₁ and R₄-R₈ are H, R₂ and R₃ are not both—CH₂C(NHC(O)CH₃)(C(O)OCH₂CH₃)₂ or —CH₂CH(NHC(O)CH₃)C(O)OCH₂CH₃; xv. whenR₃-R₇ are H and R₁ is NH₂, R₂ and R₈ are not both —SCH₂C(O)OH; xvi. whenR₃-R₈ are H, R₁ and R₂ are not both —OC(O)CH₂CH₂C(O)O CH₂CH₃; xvii. whenR₁, R₃-R₅, R₇, and R₈ are H, R₂ and R₆ are not both—OC(O)CH₂CH₂C(O)OCH₂CH₃; xx. when R₂, R₃, R₆, and R₇ are H and R₄ and R₅are —(CH₂)₅CH₃, R₁ and R₈ are not both —(CH₂)₇C(O)OH; xxii. when R₃ andR₆ are H and R₁, R₄, R₅, and R₈ are —NH₂, R₂ and R7₅ are not both—O(CH₂)₃C(O)OCH₃.
 12. The compound of claim 11, wherein X₁ and X₂ are O,and/or wherein each of R₁-R₈ is independently H; halo; optionallysubstituted C₁₋₆ alkyl; —X₁-L₁-C(O)O—Y₁; or —X₂-L₂-C(O)O—Y₂, provided R₂is —X₁-L₁-C(O)O—Y₁ or —X₂-L₂-C(O)O—Y₂, and/or wherein R₂ is—X₁-L₁-C(O)O—Y₁ and R₆ is —X₂-L₂-C(O)O—Y₂, and/or wherein Y₁ and Y₂ areH.
 13. The compound of claim 11, wherein X₃ and X₄ are O, and/or whereineach of R₁-R₈ is independently H; halo; optionally substituted C₁₋₆alkyl; —X₃-L₃-P(═O)(OY₃)₂; or —X₄-L₄-P(═O)(OY₄)₂, provided R₂ is—X₃-L₃-P(═O)(OY₃)₂, or —X₄-L₄-P(═O)(OY₄)₂, and/or wherein R₂ is—X₃-L₃-P(═O)(OY₃)₂ and R₆ is —X₄-L₄-P(═O)(OY₄)₂, and/or wherein Y₃ andY₄ are H.
 14. The compound of claim 11, wherein the compound of formulaI is

or an ion, salt, or hydroquinone thereof; wherein R₂ is —O(CH₂)₃C(O)OH.15. The compound of claim 11, wherein the compound of formula I is

or a salt, ion, or hydroquinone thereof.
 16. The compound of claim 11,wherein the compound of formula I is

or an ion, salt, or hydroquinone thereof.
 17. The compound of claim 11,wherein the compound of formula I is:

or an ion, salt, or hydroquinone thereof; wherein R₂ is—O(CH₂)₃P(O)(OH)₂.
 18. The compound of claim 11, wherein the compound offormula 1 is:

or an ion, salt, or hydroquinone thereof.